Modulation of the coffee-ring effect in particle/surfactant mixtures: the importance of particle-interface interactions

Self-assembly behavior and adhesive performance of imidazolium cation grafted cellulose nanocrystals in confined space

Confined evaporation-induced self-assembly (C-EISA) is a powerful technique to guide disordered nanoparticles into long-range organized structures. Herein, we investigate the C-EISA behavior of 1-butyl-3-vinylimidazolium cation ([VBIm]+) grafted cellulose nanocrystals (CNC-C) in a parallel-plates confined geometry. Interestingly, CNC-C can spontaneously assemble into maze-like patterns with branch dimensions on the micrometer scale and uniformly distributed throughout the confined space, which is completely different from the lamellar self-assembly patterns of unmodified CNCs. Combining in situ observations and microscopic characterization, we speculate that the formation of maze-like patterns originates from the reduction of colloidal stability induced by the grafted imidazolium cations. The electrostatic attraction between CNC-C aggregated bundles and glass substrates acts as anchor points, thereby leading to the unstable motion of the liquid-air menisci during the inward intrusion of air. Due to the physicochemical properties and unique C-EISA behavior, the CNC-C based adhesive can maintain adhesion at temperatures of ca. 200 °C, while rapidly debonding when immersed in water, demonstrating the potential to be used as stimuli-responsive temporary or removable adhesives. Furthermore, the strategy proposed in this work for achieving CNCs patterning is also promising to be extended to other anisotropic rod-shaped nanoparticles.

Quantifying the Microstructure of Dried Deposits Using Height-Height Correlation Function

Colloidal self-assembly has garnered significant attention in recent research, owing to applications in medical and engineering domains. Understanding the arrangement of particles in self-assembled systems is crucial for comprehending the underlying physics and synthesizing complex nano- and microscale structures. In this study, we introduce a novel methodology for analyzing the spatial distribution of particles in colloidal assemblies, focusing specifically on quantifying the microstructure of deposits formed by the evaporation of colloidal particle-laden drops. Utilizing a height-height correlation-function-based approach, we quantify variations in the height profile of deposits in radial and azimuthal directions. This approach enables the classification of the patterns into typical examples encountered in an evaporation-driven assembly. The method is demonstrated to be robust for quantifying synthetic and experimentally obtained deposit patterns, exhibiting excellent agreement in the estimated parameters. The mapping developed between pattern morphology and the quantitative measures introduced in this work may be used in a variety of applications including disease diagnosis as well as in developing pattern recognition tools.

Solution-Processable Route for Large-Area Uniform 2D Semiconductor Nanofilms

The semiconductor thin film engineering technique plays a key role in the development of advanced electronics. Printing uniform nanofilms on freeform surfaces with high efficiency and low cost is significant for actual industrialization in electronics. Herein, a high-throughput colloidal printing (HTCP) strategy is reported for fabricating large-area and uniform semiconductor nanofilms on freeform surfaces. High-throughput and uniform printing rely on the balance of atomization and evaporation, as well as the introduced thermal Marangoni flows of colloidal dispersion, that suppresses outward capillary flows. Colloidal printing with in situ heating enables the fast fabrication of large-area semiconductor nanofilms on freeform surfaces, such as SiO2 /Si, Al2 O3 , quartz glass, poly(ethylene terephthalate) (PET), Al foil, plastic tube, and Ni foam, expanding their technological applications where substrates are essential. The printed SnS2 nanofilms are integrated into thin-film semiconductor gas sensors with one of the fastest responses (8 s) while maintaining the highest sensitivity (Rg /Ra = 21) (toward 10 ppm NO2 ), as well as an ultralow limit of detection (LOD) of 46 ppt. The ability to print uniform semiconductor nanofilms on freeform surfaces with high-throughput promises the development of next-generation electronics with low cost and high efficiency.

Versatile Deposition of Complex Colloidal Assemblies from the Evaporation of Hanging Drops

Existing strategies designed to produce ordered arrangements of colloidal particles on solid supports are of great interest for their wide range of applications, from colloidal lithography, plasmonic and biomimetic surfaces to tags for anti-counterfeiting, but they all share various degrees of complexity hampering their facile implementation. Here, a drastically simplified methodology is presented to achieve ordered particle deposition, consisting in adding micromolar amounts of cationic surfactant to a colloidal suspension drop and let it evaporate in an upside-down configuration. Confinement at the air/water interface enables particle assembly into monolayers, which are then transferred on the substrate producing highly ordered structures displaying vivid, orientation-dependent structural colors. The method is compatible with many particle types and substrates, while controlling system parameters allows tuning the deposit size and morphology, from monocrystals to polycrystalline disks and "irises", from single-component to crystal alloys with Moiré patterns, demonstrating its practicality for a variety of processes.

Recent advances in whiskey analysis for authentication, discrimination, and quality control

In order to safeguard authentic whiskey products from fraudulent or counterfeit practices, high throughput solutions that provide robust, rapid, and reliable solutions are required. The implementation of some analytical strategies is quite challenging or costly in routine analysis. Qualitative screening of whiskey products has been explored, but due to the nonspecificity of the chemical compounds, a more quantitative confirmatory technique is required to validate the result of the whiskey analysis. Hence, combining analytical and chemometric methods has been fundamental in whiskey sample differentiation and classification. A comprehensive update on the most relevant and current analytical techniques, including spectroscopic, chromatographic, and novel technologies employed within the last 5 years in whiskey analysis for authentication, discrimination, and quality control, are presented. Furthermore, the technical challenges in employing these analytical techniques, future trends, and perspectives are emphasized.

In Situ Interfacial-Assembly Perovskite Quantum Dot via Marangoni and Capillary Convection Manipulation for Robust Luminescence

In solid substrates, colloidal solutions produce irregular deposits on the surface by Marangoni flow and capillary flow during evaporation. Reportedly, perovskite quantum dots (PQDs) as a colloidal solution have irregular surfaces based on a similar principle as the coffee ring effect in QD systems when droplets evaporate from the substrate. Given that this issue is due to the direction of Marangoni and capillary flows, the substrate is tilted to change the direction of the flows. The appropriate angle is determined by controlling the angle of the substrate so that the two flows circulate similarly; this method is called "assembly-coating". Herein, we compare the PL intensity before and after the thermal evaporation of the thin films prepared by conventional and assembly-coating. Moreover, by characterizing the diode device (hole-only space charge limited current) for each coating process, the charge carrier characteristics are investigated in detail. Therefore, we suggest a facile strategy to obtain a uniform surface and thermal evaporative stability using colloidal solutions. This strategy is effective in designing surface uniformity and light-emitting layers for colloidal solution deposition and assembly.

Depletion zone in two-dimensional deposits of soft microgel particles

HYPOTHESIS

Microgels are a class of model soft colloids that act like surfactants due to their amphiphilicity and are spontaneously adsorbed to the fluid-air interface. Here, we exploit the surfactant-like characteristics of microgels to generate Marangoni stress-induced fluid flow at the surface of a drop containing soft colloids. This Marangoni flow combined with the well-known capillary flow that arises during the evaporation of a drop placed on a solid surface, leads to the formation of a novel two-dimensional deposit of particles with distinct depletion zones at its edge.

EXPERIMENTS

The evaporation experiments using sessile and pendant drops containing microgel particles were carried out, and the microstructure of the final particulate deposits were recorded. The kinetics of the formation of the depletion zone and its width is studied by tracking the time evolution of the microgel particle monolayer adsorbed to the interface using in situ video microscopy.

FINDINGS

The experiments reveal that the depletion zone width linearly increases with droplet volume. Interestingly, the depletion zone width is larger for drops evaporated in pendant configuration than the sessile drops, which is corroborated by considering the gravitational forces exerted on the microgel assembly on the fluid-air interface. The fluid flows arising from Marangoni stresses and the effect of gravity provide novel ways to manipulate the self-assembly of two-dimensional layers of soft colloids.

Simultaneous quantification of serum albumin and gamma globulin using Zn(II)-metallosurfactant via a coffee ring pattern

Herein, we report interactivity and conjugate formation ability between a Zn(II)-metallosurfactant and two clinically relevant serum proteins, albumin (ALB) and γ-globulin (GGB). We found that the surfactant-ALB conjugate promotes coffee ring formation, whereas with GGB it gets suppressed, which is due to the difference in structural anisotropy and hydrophobicity of the conjugates. Additionally, validation of this biosensing platform has been established in human serum samples, and it has potential applications for on-spot rapid diagnostics in remote areas.

Anisotropic nanocluster arrays to a diminished zone: different regimes of surface deposition of gold nanocolloids

Evaporation-induced assembly of nanoparticles has emerged as a versatile technique for the production of large-scale ordered structures and materials with complex features. In this study, we show that a dried particulate of an anisotropic nanocolloid undergoes non-ubiquitous surface morphological transitions at varying particle concentrations. Below 5 nM, deposits reveal the formation of linear arrays of AuNR clusters outside of the coffee ring and an annular CTAB-rich depletion zone in the inner vicinity of the coffee ring. For nanoparticle concentrations ≥5 nM, the outer cluster deposits disappear and a region of reduced AuNR density, sandwiched between the coffee ring and the depletion zone, analogous to the diminished zone, is observed. Within the coffee-ring deposits, nanoscale smectic AuNR assembly occurs via the expulsion of the cetyltrimethyl ammonium bromide (CTAB) bilayer, which contributes to the inward solutal Marangoni flow. An enhanced inward solutal Marangoni flow at high particle concentrations assists in the formation of a wider depletion zone, the emergence of the diminished zone and suppression of the width of the coffee-ring deposits. Through detailed analysis of data from ex situ (scanning electron microscopy, SEM) and in situ (contact angle and confocal imaging) measurements, we establish a direct correlation between the different evaporation modes and the various deposition regimes. A detailed mechanism for the surface morphology modulation of AuNR deposits by tuning the nanoparticle concentration in the drying sessile drop is discussed.

The control of dry-out patterns using bubble-containing droplets

HYPOTHESIS

For an evaporating nanofluid droplet that contains a bubble inside, we suspect the bubble boundary remains pinned during evaporation whereas the droplet perimeter recedes. Thus, the dry-out patterns are mainly determined by the presence of the bubble and their morphology can be tuned by the size and location of the added bubble.

EXPERIMENTS

Bubbles with varying base diameters and lifetimes are added into evaporating droplets that contain nanoparticles with different types, sizes, concentrations, shapes, and wettability. The geometric dimensions of the dry-out patterns are measured.

FINDINGS

For a droplet containing a long-lifetime bubble, a complete ring-like deposit forms, and its diameter and thickness increases and decreases with the bubble base diameter, respectively. The ring completeness, i.e., the ratio of actual ring length to its imaginary perimeter, decreases with the decrease in bubble lifetime. The pinning of droplet receding contact line by particles near the bubble perimeter has been found to be the key factor leading to ring-like deposits. This study introduces a strategy of producing ring-like deposit and allows a control of the ring morphology in a simple, cheap, and impurity-free fashion, which is applicable to various applications associated with evaporative self-assembly.

Connecting Colloidal Forces to the Equilibrium Thickness of Particulate Deposits on a Substrate in Contact with a Suspension Using Classical Density Functional Theory

We study contributions of colloidal forces, i.e., hydrophobic, van der Waals, and electrical double layer interactions, to the thickness of a colloidal deposit in equilibrium with an aqueous suspension by using classical density functional theory, which we expand to obtain a Ginzburg-Landau type energy functional. We regard colloidal particles as clusters of molecular segments-a reminiscent of polymer statistical physics and of the classic Hamaker treatment of van der Waals interactions between particles. This approach appropriately accounts for the integral interaction energy between colloidal particles, which may take magnitudes of many times the characteristic molecular thermal energy k_BT (Boltzmann constant times temperature). The analysis highlights the well-known insight that entropy is mostly governed by the solvent molecules and gives physical values to the statistical coefficients in a Ginzburg-Landau type energy functional.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Orientation-Controlled Ultralong Assembly of Janus Particles Induced by Bubble-Driven Instant Quasi-1D Interfaces

Constructing precisely oriented assemblies and exploring their orientation-dependent properties remain a challenge for Janus nanoparticles (JNPs) due to their asymmetric characteristics. Herein, we propose a bubble-driven instant quasi-1D interfacial strategy for the oriented assembly of JNP chains in a highly controllable manner. It is found that the rapid formation of templated bubbles can promote the interfacial orientation of JNPs kinetically, while the confined quasi-1D interface in the curved liquid bridge can constrain the disordered rotation of the particles, yielding well-oriented JNP chains in a long range. During the evaporation process, the interfacial orientation of the JNPs can be transferred to the assembled chains. By regulating the amphiphilicity of the JNPs, both heteraxial and coaxial JNP assemblies are obtained, which show different polarization dependences on light scattering, and the related colorimetric logic behaviors are demonstrated. This work demonstrates the great potential of patterned interfacial assembly with a manageable orientation and shows the broad prospect of asymmetric JNP assembly in constructing novel optoelectronic devices.

Printing Crack-Free Microporous Structures by Combining Additive Manufacturing with Colloidal Assembly

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle-templates, how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"-strategy is introduced to print macroscale, crack-free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near-infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal-based printed structures.

Recyclable Nonionic-Anionic Bola Surfactant as a Stabilizer of Size-Controllable and pH-Responsive Pickering Emulsions

A novel nonionic-anionic Bola surfactant (abbreviated as CH₃O(EO)₇-R₁₁-COOH) was designed and synthesized by condensation of methyl polyoxyethylene (7) ether with 12-bromododecanoic acid. In neutral aqueous solution, the surfactant behaves as a nonionic one and can stabilize oil-in-water (O/W) conventional emulsions alone and costabilize O/W Pickering emulsions with positively charged alumina nanoparticles with n-decane as the oil. In alkaline solution, the carboxylic acid group is deprotonated, becoming anionic and the surfactant is converted to Bola form, which is an inferior emulsifier and does not adsorb on particle surfaces, resulting in demulsification of both kinds of emulsions. With strong hydrophilicity, both the Bola surfactant and the bare particles return to the aqueous phase after demulsification, which is therefore recyclable and reusable in accordance with sustainable chemistry and engineering. In acidic media between pH 3 and 6, the ethyleneoxy groups tend to desorb from particle surfaces, slightly reducing the hydrophobicity of the particles. However, Pickering emulsions are still stable but their droplet size increases on lowering the pH. The Pickering emulsions are therefore pH-responsive and size-controllable. This newly designed Bola surfactant is effective in preparing smart emulsions, which are extensively applied in heterogeneous catalysis, oil product transportation, emulsion polymerization, and new material preparation.

Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications

The uneven deposition at the edges of an evaporating droplet, termed the coffee-ring effect, has been extensively studied during the past few decades to better understand the underlying cause, namely the flow dynamics, and the subsequent patterns formed after drying. The non-uniform evaporation rate across the colloidal droplet hampers the formation of a uniform and homogeneous film in printed electronics, rechargeable batteries, etc., and often causes device failures. This review aims to highlight the diverse range of techniques used to alleviate the coffee-ring effect, from classic methods such as adding chemical additives, applying external sources, and manipulating geometrical configurations to recently developed advancements, specifically using bubbles, humidity, confined systems, etc., which do not involve modification of surface, particle or liquid properties. Each of these methodologies mitigates the edge deposition via multi-body interactions, for example, particle–liquid, particle-particle, particle–solid interfaces and particle–flow interactions. The mechanisms behind each of these approaches help to find methods to inhibit the non-uniform film formation, and the corresponding applications have been discussed together with a critical comparison in detail. This review could pave the way for developing inks and processes to apply in functional coatings and printed electronic devices with improved efficiency and device yield.

Effect of the Dispersion Process and Nanoparticle Quality on Chemical Sensing Performance

On the surface of chemiresistive films, the scarce heterogeneity of a molecularly capped gold nanoparticle (MCGNP) colloidal dispersion and uneven evaporation of the MCGNP-contained drying drop applied to this surface are among the main factors that affect reproducibility, and repeatable fabrication of thin films of MCGNPs. This article shows that an increase in reproducibility and repeatability is possible using a dispersant and a surfactant during the deposition and annealing processes of the MCGNP. The results show higher sensitivity and accuracy of the sensors for the detection of volatile organic compounds in air and an increased limit of detection. These simple and practical additions might serve as a launching pad for fabrication of other types of thin-film-based sensors.

Versatile strategy for homogeneous drying patterns of dispersed particles

After spilling coffee, a tell-tale stain is left by the drying droplet. This universal phenomenon, known as the coffee ring effect, is observed independent of the dispersed material. However, for many technological processes such as coating techniques and ink-jet printing a uniform particle deposition is required and the coffee ring effect is a major drawback. Here, we present a simple and versatile strategy to achieve homogeneous drying patterns using surface-modified particle dispersions. High-molecular weight surface-active polymers that physisorb onto the particle surfaces provide enhanced steric stabilization and prevent accumulation and pinning at the droplet edge. In addition, in the absence of free polymer in the dispersion, the surface modification strongly enhances the particle adsorption to the air/liquid interface, where they experience a thermal Marangoni backflow towards the apex of the drop, leading to uniform particle deposition after drying. The method is independent of particle shape and applicable to a variety of commercial pigment particles and different dispersion media, demonstrating the practicality of this work for everyday processes.

Coating technologies call for effective methods capable of suppressing the coffee-ring effect for a uniform particle deposition. Rey et al. show homogeneous drying patterns can be achieved via physically adsorbing polymers onto particle surfaces and the method is applicable to a wide range of materials regardless of the shape of the dispersed particles.

Rotating Micro-Spheres for adsorption monitoring at a fluid interface

HYPOTHESIS

A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials.

EXPERIMENTS

Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct description of the adsorption kinetics, while microscopy monitoring of particle acceleration\deceleration informs about the adsorption\desorption dynamics. In this work, the proposed strategy is applied at an air/water interface loaded with spherical magnetic particles negatively charged, forced to rotate by the action of a rotating magnetic field.

FINDINGS

The proposed method enables the adsorption/desorption dynamics to be followed during the earliest phase of the process, when desorption of a small fraction of particles is detected, as well as to estimate approximated values of the adsorption/desorption constants. The results obtained show that the addition of a monovalent salt or a cationic (anionic) surfactant promotes (inhibits) both adsorption and formation of permanent bonds between particles.

Effect of Particle Interactions on the Assembly of Drying Colloidal Mixtures

The effects of particle interactions on the size segregation and assembly of colloidal mixtures during drying were investigated. A cationic surfactant was added to a binary latex/silica colloidal dispersion that has been shown to self-stratify upon drying at room temperature. Atomic force microscopy was used to show that the change in particle interactions due to the presence of surfactants reduced the degree of stratification and, in some cases, suppressed the effect altogether. Colloidal dispersions containing higher surfactant concentrations can undergo a complete morphology change, resulting instead in the formation of armored particles consisting of latex particles coated with smaller silica nanoparticles. To further prove that armored particles are produced and that stratification is suppressed, cross-sectional images were produced with energy-dispersive X-ray spectroscopy and confocal fluorescence microscopy. The growth of armored particles was also measured using dynamic light scattering. To complement this research, Brownian dynamics simulations were used to model the drying. By tuning the particle interactions to make them more attractive, the simulations showed the presence of armored particles, and the size segregation process was hindered. The prevention of segregation also results in enhanced transparency of the colloidal films. Overall, this research proves that there is a link between particle interactions and size segregation in drying colloidal blends and provides a valuable tool to control the assembly of different film architectures using an extremely simple method.

Challenges, Prospects, and Emerging Applications of Inkjet-Printed Electronics: A Chemist's Point of View

Driven by the development of new functional inks, inkjet-printed electronics has achieved several milestones upon moving from the integration of simple electronic elements (e.g., temperature and pressure sensors, RFID antennas, etc.) to high-tech applications (e.g. in optoelectronics, energy storage and harvesting, medical diagnosis). Currently, inkjet printing techniques are limited by spatial resolution higher than several micrometers, which sets a redhibitorythreshold for miniaturization and for many applications that require the controlled organization of constituents at the nanometer scale. In this Review, we present the physico-chemical concepts and the equipment constraints underpinning the resolution limit of inkjet printing and describe the contributions from molecular, supramolecular, and nanomaterials-based approaches for their circumvention. Based on these considerations, we propose future trajectories for improving inkjet-printing resolution that will be driven and supported by breakthroughs coming from chemistry. Please check all text carefully as extensive language polishing was necessary. Title ok? Yes.

Visualization and (Semi-)quantification of submicrometer plastics through scanning electron microscopy and time-of-flight secondary ion mass spectrometry

Increasing numbers of studies have demonstrated the existence of nanoplastics (1-999 nm) in the environment and commercial products, but the current technologies for detecting and quantifying nanoplastics are still developing. Herein, we present a combination of two techniques, e.g., scanning electron microscopy (SEM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to analyze submicron-sized plastics. A drop-casting of a 20-nL particle suspension on a Piranha solution-cleaned silicon wafer with dry ice incubation and subsequent freeze-drying was used to suppress the coffee-ring effect. SEM images were used to quantify particles, and this technique is applicable for 0.195-1.04-μm polystyrene (PS), 0.311-μm polyethylene terephthalate (PET), and 0.344-μm polyethylene (PE) at a minimum concentration of 2.49 × 10⁹ particles/mL. ToF-SIMS could not quantify the particle number, while it could semi-quantitatively estimate number ratios of submicron PE, PET, polyvinyl chloride (PVC), and PS particles in the mixture. Analysis of submicron plastics released from three hot water-steeped teabags (respectively made of PET/PE, polylactic acid (PLA), and PET) was revisited. The SEM-derived sizes and particle numbers were comparable to those measured by a nanoparticle tracking analysis (NTA) regardless of whether or not the hydro-soluble oligomers were removed. ToF-SIMS further confirmed the number ratios of different particles from a PET/PE composite teabag leachate. This method shows potential for application in analyzing more-complex plastic particles released from food contact materials.

Inkjet-Patterned Microdroplets as Individual Microenvironments for Adherent Single Cell Culture

Adhesion of single cells is the foundation of manifold cellular behaviors and life processes. However, investigating the function of a specific cell is still challenging due to deficiency of adhesion or interference from surrounding cells. Herein, an open microfluidic system is reported for culturing adherent single cells, implemented by a micrometer-scale droplet matrix on an inkjet-printed polylysine template. The target cells are isolated from any cell from other droplets, and their adhesion strength is determined to be comparable to conventional petri dishes via an in-situ investigation with a microfluidic extractor. On this proposed platform, isolated single cells are observed to display an entirely distinct spreading behavior featuring total absence of elongation, indicating drastic cell behavior change from their "singleness." This system has high versatility and compatibility for various assaying methods, assuring a promising potential in detailed single cell behavior and cell heterogeneity studies.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Influence of fluid flows on electric double layers in evaporating colloidal sessile droplets

A model is developed for describing the transport of charged colloidal particles in an evaporating sessile droplet on the electrified metal substrate in the presence of a solvent flow. The model takes into account the electric charge of colloidal particles and small ions produced by electrolytic dissociation of the active groups on the colloidal particles and solvent molecules. We employ a system of self-consistent Poisson and Nernst-Planck equations for electric potential and average concentrations of colloidal particles and ions with the appropriate boundary conditions. The fluid dynamics, temperature distribution and evaporation process are described with the Navier-Stokes equations, equations of heat conduction and vapor diffusion in air, respectively. The developed model is used to carry out a first-principles numerical simulation of charged silica colloidal particle transport in an evaporating aqueous droplet. We find that electric double layers can be destroyed by a sufficiently strong fluid flow.

Long-Range-Ordered Assembly of Micro-/Nanostructures at Superwetting Interfaces

On-chip integration of solution-processable materials imposes stringent and simultaneous requirements of controlled nucleation and growth, tunable geometry and dimensions, and long-range-ordered assembly, which is challenging in solution process far from thermodynamic equilibrium. Superwetting interfaces, underpinned by programmable surface chemistry and topography, are promising for steering transport, dewetting, and microfluid dynamics of liquids, thus opening a new paradigm for micro-/nanostructure assembly in solution process. Herein, assembly methods on the basis of superwetting interfaces are reviewed for constructing long-range-ordered micro-/nanostructures. Confined capillary liquids, including capillary bridges and capillary corner menisci realized by controlling local wettability and surface topography, are highlighted for simultaneously attained deterministic patterning and long-range order. The versatility and robustness of confined capillary liquids are discussed with assembly of single-crystalline micro-/nanostructures of organic semiconductors, metal-halide perovskites, and colloidal-nanoparticle superlattices, which lead to enhanced device performances and exotic functionalities. Finally, a perspective for promising directions in this realm is provided.

Patterning of Metallic Nanoparticles over Solid Surfaces from Sessile Droplets by Thermoplasmonically Controlled Liquid Flow

Optically controlled assembly of suspended particles from evaporating sessile droplets is an emerging method to realize on-demand patterning of particles over solid substrates. Most of the reported strategies rely either on additives or surface texturing to modulate particle deposition. Though dynamic control over the assembly of microparticles is possible, limited success has been achieved in nanoparticle patterning, especially in the case of metallic nanoparticles. This work demonstrates a simple light-directed patterning of gold (Au) nanoparticles based on the thermoplasmonically controlled liquid flow. Excitation at the plasmonic wavelength (532 nm) generates the required temperature gradient, resulting in the particle assembly at the irradiation zone in response to the thermocapillary flow created inside the droplet. Particle streak velocimetry experiments and analysis confirm the existence of a strong thermocapillary flow, which counteracts the naturally occurring evaporative convection flows. By modulating the illumination conditions, we could achieve patterns with various morphologies, including center deposit, off-center deposit, multi-spot deposit, and lines. We successfully applied the developed strategy for realizing closely packed hybrid particle assembly containing different particles: Au and polystyrene particles (PS). We performed optical microscopy, 3D profilometry, and SEM analysis to characterize the particle deposit. We analyzed the periodicity of Au-PS hybrid assembly using fast Fourier transform and radial distribution function analysis. PS particles formed a hexagonal close-packed arrangement at the irradiation zone, with Au NPs residing inside the voids. We believe that the presented strategy could significantly enhance the applicability of the evaporative lithography from sessile droplets for the programmable patterning of metallic nanoparticles.

Redox Control of Particle Deposition from Drying Drops

The coffee-ring effect (CRE), which denotes the accumulation of nonvolatile compounds at the periphery of a pinned sessile drying drop, is a universal and ubiquitous yet complex phenomenon. It is crucial to better understand and control it, either to avoid its various deleterious consequences in many processes requiring homogeneous deposition or to exploit it for applications ranging from controlled particle patterning to low cost diagnostics. Here, we report for the first time the use of a reduction-oxidation (redox) stimulus to cancel the CRE or harness it, leading to a robust and tunable control of particle deposition in drying sessile drops. This is achieved by implementing redox-sensitive ferrocenyl cationic surfactants of different chain lengths in drying drops containing anionic colloids. Varying surfactant hydrophobicity, concentration, and redox state allows us not only to control the overall distribution of deposited particles, including the possibility to fully cancel the CRE, but also to modify the microscopic organization of particles inside the deposit. Notably, with all other parameters being fixed, this method allows the adjustment of the deposited particle patterns, from polycrystalline rings to uniform disks, as a function of the oxidation rate. We show that the redox control can be achieved either chemically by the addition of oxidants or electrochemically by applying a potential for additive-free and reversible actuation in a closed system. This correlation between the redox state and the particle pattern opens a perspective for both redox-programmable particle patterning and original diagnostic applications based on the visual determination of a redox state. It also contributes to clarify the role of surfactant charge and its amphiphilic character in directing particle deposition from drying suspensions.

Controllable dried patterns of colloidal drops

HYPOTHESIS

When an aqueous colloidal drop dries on a solid substrate, the final pattern of the dried deposit can be manipulated through controlling the internal flow states of the drop.

EXPERIMENTS

We report a strategy to control the dried patterns of aqueous colloidal drop by controlling the drop configurations and relative humidity. For this purpose, both sessile and pendant drops are studied.

FINDING

The capillary flow, which is responsible for coffee-ring, is suppressed by increasing the relative humidity. Then, surprisingly, the internal convection in the pendant drop is significantly stronger than that in the sessile drop. This phenomenon leads to the formation of the disc-like and spot-like dried patterns in the sessile and pendant drop, respectively, which are the results of different interactions between the Marangoni and (buoyancy-induced) natural convections in the sessile and pendant drops. In the sessile drop, the Marangoni and natural convections mutually restrain each other due to their opposite flow directions. In contrast, in the pendant drop, the two convections mutually enhance each other, due to their same flow directions. Thisnew strategy offers a foreign-material-free and external-force-free means to control the dried patterns of the drop.

Surface Properties of Colloidal Particles Affect Colloidal Self-Assembly in Evaporating Self-Lubricating Ternary Droplets

In this work, we unravel the role of surface properties of colloidal particles on the formation of supraparticles (clusters of colloidal particles) in a colloidal Ouzo droplet. Self-lubricating colloidal Ouzo droplets are an efficient and simple approach to form supraparticles, overcoming the challenge of the coffee stain effect in situ. Supraparticles are an efficient route to high-performance materials in various fields, from catalysis to carriers for therapeutics. Yet, the role of the surface of colloidal particles in the formation of supraparticles using Ouzo droplets remains unknown. Therefore, we used silica particles as a model system and compared sterically stabilized versus electrostatically stabilized silica particles─positively and negatively charged. Additionally, we studied the effect of hydration. Hydrated negatively charged silica particles and sterically stabilized silica particles form supraparticles. Conversely, dehydrated negatively charged silica particles and positively charged amine-coated particles form flat film-like deposits. Notably, the assembly process is different for all the four types of particles. The surface modifications alter (a) the contact line motion of the Ouzo droplet and (b) the particle-oil and particle-substrate interactions. These alterations modify the particle accumulation at the various interfaces, which ultimately determines the shape of the final deposit. Thus, by modulating the surface properties of the colloidal particles, we can tune the shape of the final deposit, from a spheroidal supraparticle to a flat deposit. In the future, this approach can be used to tailor the supraparticles for applications such as optics and catalysis, where the shape affects the functionality.

Flow profiles near receding three-phase contact lines: influence of surfactants

The dynamics of wetting and dewetting is largely determined by the velocity field near the contact lines. For water drops it has been observed that adding surfactant decreases the dynamic receding contact angle even at a concentration much lower than the critical micelle concentration (CMC). To better understand why surfactants have such a drastic effect on drop dynamics, we constructed a dedicated setup on an inverted microscope, in which an aqueous drop is held stationary while the transparent substrate is moved horizontally. Using astigmatism particle tracking velocimetry, we track the 3D displacement of the tracer particles in the flow. We study how surfactants alter the flow dynamics near the receding contact line of a moving drop for capillary numbers in the order of 10^-6. Even for surfactant concentrations c far below the critical micelle concentration (c ≪ CMC) Marangoni stresses change the flow drastically. We discuss our results first in a 2D model that considers advective and diffusive surfactant transport and deduce estimates of the magnitude and scaling of the Marangoni stress from this. Modeling and experiment agree that a tiny gradient in surface tension of a few μN m^-1 is enough to alter the flow profile significantly. The variation of the Marangoni stress with the distance from the contact line suggests that the 2D advection-diffusion model has to be extended to a full 3D model. The effect is ubiquitous, since surfactant is present in many technical and natural dewetting processes either deliberately or as contamination.

Manipulating Nano-suspension Droplet Evaporation by Particle Surface Modification

The evaporation of a nano-suspension droplet on a substrate has gained extensive attention recently due to its potential application in the rising industry of functional coating. In this paper, we reported that the droplet evaporation behavior can be controlled by the nanoparticles' concentration and the functional group on the surface of nanoparticles. Experimental results indicated that the suspension of nanoparticles notably enhanced the evaporation rate of droplets and decreased the duration time of the continuous-contact-radius (CCR) stage. This effect was more obvious when the nanoparticles were modified by the perfluorodecyltrimethoxysilane (PDTS), which made the particles more hydrophobic. Besides, the modified nanoparticles can effectively inhibit the formation of coffee rings during evaporation. These results may have important applications for the energy-efficient enhancement of the water evaporation rate.

pH-Responsive Behavior of Pickering Emulsions Stabilized by a Selenium-Containing Surfactant and Alumina Nanoparticles

Herein, we describe pH-responsive Pickering emulsions stabilized by a sodium carboxylate-derived selenium surfactant (C₁₀-Se-C₁₀·(COONa)₂) in combination with positively charged alumina nanoparticles. Unlike other bola-type carboxylate surfactants (e.g., disodium eicosanoate), C₁₀-Se-C₁₀·(COONa)₂ is soluble in water with a low Krafft temperature (36.1 °C). The emulsions are sensitive to pH variations, and efficient demulsification can be achieved by a pH trigger. The carboxylic sodium group in the C₁₀-Se-C₁₀·(COONa)₂ structure can be reversibly cycled between its anionic and nonionic states (carboxylic acid), resulting in a pH-controlled electrostatic attraction between the surfactant and alumina. The Pickering emulsion can be reversibly switched between "on" (stable) and "off" (unstable) states by pH at least four times. Compared with the emulsions stabilized by specially synthesized stimuli-responsive particles or surfactants, the method reported here is much easier to implement and requires very low concentrations of the surfactant and nanoparticles, with potential applications in the fields of biomedicine, drug delivery, and cosmetics.

Exploiting Additives for Directing the Adsorption and Organization of Colloid Particles at Fluid Interfaces

The self-assembly of colloids at fluid interfaces is a well-studied research field both for gaining fundamental insights and for material fabrication. The fluid interface allows the confinement of particles in two dimensions and may act as a template for guiding their organization into soft and reconfigurable structures. Additives (e.g., surfactants, salts, and polymers) in the colloidal suspension are routinely used as a practical and effective tool to drive particle adsorption and tune their interfacial organization. However, some phenomena lying at the heart of the accumulation and self-assembly of particles at fluid interfaces remain poorly understood. This Feature Article aims to critically analyze the mechanisms involved in the adsorption and self-organization of micro- and nanoparticles at various fluid interfaces. In particular, we address the role of additives in both promoting the adsorption of particles from the bulk suspension to the fluid interface and in mediating the interactions between interfacial particles. We emphasize how different types of additives play a crucial role in controlling the interactions between suspended particles and the fluid interface as well as the interactions between adsorbed particles, thus dictating the final self-assembled structure. We also critically summarize the main experimental protocols developed for the complete adsorption of particles initially suspended in the bulk. Furthermore, we highlight some special properties (e.g., reconfigurability upon external stimulation and dissipative self-assembly) and the application potential of structures formed by colloid self-organization at fluid interfaces mediated/promoted by additives. We believe our contribution serves both as a practical roadmap to scientists coming from other fields and as a valuable information resource for all researchers interested in this exciting research field.

Phase behaviour in 2D assemblies of dumbbell-shaped colloids generated under geometrical confinement

The structure formation and the phase behaviour of monolayers of dumbbell-shaped colloids are explored. For this, we conduct Langmuir-Blodgett experiments at the air/water interface and conventional Brownian dynamic simulations without hydrodynamic interactions. Using Voronoi tessellations and the probability density of the corresponding shape factor of the Voronoi cells p(ζ), the influence of the area fraction φ on the structure of the monolayers is investigated. An increase of the area fraction leads to a higher percentage of domains containing particles with six nearest neighbours and a sharper progression of p(ζ). Especially in dense systems, these domains can consist of aligned particles with uniform Voronoi cells. Thus, the increase of φ enhances the order of the monolayers. Simulations show that a sufficient enhancement of φ also impacts the pair correlation function which develops a substructure in its first maxima. Furthermore, we find that reducing the barrier speed in the Langmuir-Blodgett experiments enhances the final area fraction for a given target surface pressure which, in turn, also increases the percentage of particles with six nearest neighbours and sharpens the progression of p(ζ). Overall, the experiments and simulations show a remarkable qualitative agreement which indicates a versatile way of characterising colloidal monolayers by Brownian dynamics simulations. This opens up perspectives for application to a broad range of nanoparticle-based thin film coatings and devices.

Particle-laden fluid/fluid interfaces: physico-chemical foundations

Particle-laden fluid/fluid interfaces are ubiquitous in academia and industry, which has fostered extensive research efforts trying to disentangle the physico-chemical bases underlying the trapping of particles to fluid/fluid interfaces as well as the properties of the obtained layers. The understanding of such aspects is essential for exploiting the ability of particles on the stabilization of fluid/fluid interface for the fabrication of novel interface-dominated devices, ranging from traditional Pickering emulsions to more advanced reconfigurable devices. This review tries to provide a general perspective of the physico-chemical aspects associated with the stabilization of interfaces by colloidal particles, mainly chemical isotropic spherical colloids. Furthermore, some aspects related to the exploitation of particle-laden fluid/fluid interfaces on the stabilization of emulsions and foams will be also highlighted. It is expected that this review can be used for researchers and technologist as an initial approach to the study of particle-laden fluid layers.

Texture Analysis of Dried Droplets for the Quality Control of Medicines

The quality control of medicines guarantees the effectiveness of treatments for diseases. We explore the use of texture analysis of patterns in dried droplets as a tool to readily detect both impurities and changes in drug concentration. Four types of medicines associated with different routes of administration were analyzed: Methotrexate, Ciprofloxacin, Clonazepam, and Budesonide. We use NaCl and a hot substrate at 63 ∘C to promote aggregate formation and to reduce droplet drying time. Depending on the medicine, optical microscopy reveals different complex aggregates such as circular to oval splatters, fern-like islands, crown shapes, crown needle-like and bump-like patterns as well as dendritic branched and star-like crystals. We use some physical features of the stains (as the stain diameter and superficial area) and gray level co-occurrence matrix (GLCM) to characterize patterns of dried droplets. Finally, we show that structural analysis of stains can achieve 95% accuracy in identifying medicines with 30% water dilution, while it achieves 99% accuracy in detecting drugs with 10% other substances.

Fabricating and printing chemiresistors based on monolayer-capped metal nanoparticles

Chemiresistors that are based on monolayer-capped metal nanoparticles (MCNPs) have been used in a wide variety of innovative sensing applications, including detection and monitoring of diagnostic markers in body fluids, explosive materials, environmental contaminations and food quality control. The sensing mechanism is based on reversible swelling or aggregation and/or changes in dielectric constant of the MCNPs. In this protocol, we describe a procedure for producing MCNP-based chemiresistive sensors that is reproducible from device to device and from batch to batch. The approach relies on three main steps: (i) controlled synthesis of gold MCNPs, (ii) fabrication of electrodes that are surrounded with a microbarrier ring to confine the deposited MCNP solution and (iii) a tailor-made drying process to enable evaporation of solvent residues from the MCNP sensing layer to prevent a coffee-ring effect. Application of this approach has been shown to produce devices with ±1.5% variance-a value consistent with the criterion for commercial sensors-as well as long shelf life and stability. Fabrication of chemical sensors based on dodecanethiol- or 2-ethylhexanethiol-capped MCNPs with this approach provides high sensitivity and accuracy in the detection of volatile organic compounds (e.g., octane and decane), toxic gaseous species (e.g., HCl and NH₃) in air and simulated mixtures of lung and gastric cancer from exhaled breath.

Recent Advances in High-Throughput Nanomaterial Manufacturing for Hybrid Flexible Bioelectronics

Hybrid flexible bioelectronic systems refer to integrated soft biosensing platforms with tremendous clinical impact. In this new paradigm, electrical systems can stretch and deform with the skin while previously hidden physiological signals can be continuously recorded. However, hybrid flexible bioelectronics will not receive wide clinical adoption until these systems can be manufactured at industrial scales cost-effectively. Therefore, new manufacturing approaches must be discovered and studied under the same innovative spirit that led to the adoption of novel materials and soft structures. Recent works have taken mature manufacturing approaches from the graphics industry, such as gravure, flexography, screen, and inkjet printing, and applied them to fully printed bioelectronics. These applications require the cohesive study of many disparate parts. For instance, nanomaterials with optimal properties for each specific application must be dispersed in printable inks with rheology suited to each printing method. This review summarizes recent advances in printing technologies, key nanomaterials, and applications of the manufactured hybrid bioelectronics. We also discuss the existing challenges of the available nanomanufacturing methods and the areas that need immediate technological improvements.

Wetting and Drying of Aqueous Droplets Containing Nonionic Surfactants CnEm

This paper presents a systematic study of the wetting and drying of aqueous pico-liter droplets containing nonionic surfactants polyoxyethylene alkyl ethers (C_nE_m; n = 10, 12, 14, m = 6 or 8) in comparison with the anionic surfactant sodium dodecyl sulfate (SDS). The spreading and drying of droplets on hydrophilic substrates were studied by tracking the three-phase contact line (TCL) and by interferometry. C_nE_m droplets undergo phase separation during drying: a water-rich droplet retracts and leaves behind a thin film that is postulated to be a surfactant mesophase. This thin film either retracts or breaks up into small droplets on a longer time scale. The receding contact angle of the water-rich droplet on the thin film in the late stage of drying of C_nE_m droplets is independent of hydrophobicity of substrates, supporting the inference that a mesophase is present on the surface. Both C_nE_m and SDS solutions inhibit spreading on hydrophilic surfaces, which is attributed to Marangoni contraction as a result of a surface tension gradient across the gas-liquid interface. More pronounced suppression of spreading is observed in the case of C_nE_m solutions, possibly due to the phase transition of surfactant solution in the vicinity of the initial TCL leading to a viscous phase at the TCL that pins the droplet. Tracer particle measurements reveal that mild Marangoni flows exist for droplets with surfactant concentrations well above the critical micelle concentration (CMC). Origins of the surfactant gradients that result in Marangoni flows are discussed.

Unveiling specific nanoparticle-protein interactions via evaporated drops: From molecular recognition to allergen identification

Unveiling specific interactions between nanoparticles (NPs) and proteins could benefit a better control of NPs' performance in recognition-based detection, imaging and drug delivery. Herein, we investigated the specific recognition between an aptamer modified gold nanoparticle (Apt-AuNP) and its target protein arginine kinase (AK) through a coffee-ring effect (CRE)-based approach. The evaporated droplets of the Apt-AuNP with AK featured a ring-disk-ring transition with elevated AK concentration and a disk pattern was found when the Apt was saturated by AK. Moreover, the AK concentration versus ring thickness curve below the saturation point was proved to fit in an exponential function, indicating the strong association between the Apt-AuNP and AK. In contrast, the ring thickness above the saturation point fitted in a Gompertz growth model that was similar with the Apt-AuNPs incubated with the nonspecific protein (bovine serum albumin, BSA), suggesting that AK was nonspecifically adsorbed onto the AuNPs. The impact of the specific NP-protein interaction on the translation of CRE into macroscopic patterns was further utilized to identify target food allergen AK by the Apt-AuNPs over nontarget allergens (tropomyosin, ovalbumin and β-lactoglobulin). This work provided new insight into the general NP-protein association process and demonstrated the feasibility of employing CRE as an effective tool to profile the specific interactions between NPs and proteins.

Nonenzymatic Glucose Sensors Based on Copper Sulfides: Effect of Binder-Particles Interactions in Drop-Casted Suspensions on Electrodes Electrochemical Performance

The constant progress in novel nanomaterials synthesis has contributed to the rapid development of nonenzymatic glucose sensors. For working electrodes preparation, drop casting proved to be the most convenient and thus most widely applied method. However, appropriate interpretation of obtained electrochemical signal requires in-depth knowledge of limitations related to this technique. In this study, we prepared solutions based on commonly reported polymers for nanostructures immobilization and investigated their influence on copper sulfides distribution on the electrode. Characterization of suspensions properties and behavior of particles during droplet drying revealed that nonionic polyvinylpyrrolidone (PVP) was favorable for electrodes modification with copper sulfides in comparison with Nafion and chitosan. It ensured homogeneity of the suspension as well as the uniform coverage of the electrode surface with particles, what resulted in increased active surface area and, therefore, higher signal from glucose addition. On the other hand, when cationic chitosan was used as a binder, suspensions were agglomerated and, within dry deposits, a coffee-ring effect was observed. Appropriate adjustment of material and polymer interactions led to enhanced electrode electrochemical performance.

Particle size and substrate wettability dependent patterns in dried pendant drops

The particle laden sessile drops when dried on solid surfaces under certain conditions leave a deposit pattern wherein all the particles are confined to a narrow region close to the edge of the deposit. Such patterns which often form when coffee drops dry are referred to as the coffee ring patterns or the coffee stains. Recent research points to the formation of intriguing patterns when colloidal particle laden drops are dried in configurations other than sessile mode. In this article, the combined effect of particle size and wettability of the substrate on the patterns formed by drying drops in sessile and pendant configurations is investigated via experiments. Our results demonstrate a transition from coffee ring to central dome-like deposit morphology with decrease in wettability of the substrates when drops containing 3 μm diameter particles are dried in pendent mode. A similar transition in the deposit morphology is observed with increase in the diameter of the particles in pendant drops dried on substrates of near neutral wettability (θ = 86 ± 3°). The influence of particles size, substrate wettability and drop configuration on the kinetics of deposition of particles at the three phase contact line will also be discussed. We compare our experimental observations with particle based simulations wherein the dried patterns are generated by accounting for three particle transport modes, namely, advective particle transport resulting from capillary flow, gravity driven settling of particles and particle capture by descending interface.

Toward Controlling Evaporative Deposition: Effects of Substrate, Solvent, and Solute

Understanding evaporative deposition from a colloidal suspension and on-demand control over it are important due to its industrial and biomedical applications. In particular, it is known that interactions among substrate, solute, and solvent have important consequences on evaporative depositions; however, how these are affecting the deposition patterns and at which conditions these interactions are prominent need detailed investigations. Here we report that the total time of deposition (t_d) and the geometric shape of the droplet (L_c = initial footprint diameter/height) have a significant role in determining the evaporative deposition patterns. We have identified four zones based on t_d and L_c, and found that with longer deposition time (high t_d) and larger available space for particle motion within a liquid droplet (high L_c), deposition patterns were governed by the interactions among the substrate, solute, and solvent. We also experimentally demonstrated that the pinned contact line is indispensable for the "coffee ring" effect by comparing the deposition on surfaces with and without hysteresis. The effect of the Marangoni flow is also discussed, and it is shown that by controlling Marangoni flow, one can manipulate the droplet deposition from uniform disk-like to coffee ring with a central deposition.

Single Droplet Assembly for Two-Dimensional Nanosheet Tiling

Recent advances in two-dimensional (2D) materials offer an opportunity for atomic layer engineering of functional thin films and superlattices. For future applications of 2D materials, there is an urgent need to develop convenient deposition processes that enable precise control of thin-film architectures while reducing the time, cost, and energy/sample consumption. Here, we demonstrate a strategy for nanosheet assembly using a simple drop casting with a pipet and a hot plate. By controlled thermal convection of a single droplet on a hot plate, a range of 2D nanosheets, such as Ti_0.87O₂^0.52-, Ca₂Nb₃O₁₀^-, Ru_0.95O₂^0.2-, and graphene oxide, can be neatly tiled to form an ideal monolayer on various substrates in ∼30 s over a wide area (i.e., a 50 mmϕ substrate). The mechanism and control strategies are discussed. We also demonstrate the production of various functional coatings such as conducting, semiconducting, insulating, magnetic, and photochromic coatings in multilayer, superlattice, and submicrometer-thick forms, offering the potential for a convenient way to produce high-quality 2D nanosheet films.