Evaporative Optical Marangoni Assembly: Tailoring the Three-Dimensional Morphology of Individual Deposits of Nanoparticles from Sessile Drops

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.

Thermal Marangoni trapping driven by laser absorption in evaporating droplets for particle deposition

Controlling the deposition of particles is of great importance in many applications. In this work, we study particle deposition driven by Marangoni flows, triggered by laser absorption inside an evaporating droplet. When the laser is turned on, thermal gradients are generated and produce a toroidal Marangoni flow that concentrates the particles around the laser beam and ultimately controls the final deposition. We experimentally characterize the radius of the Marangoni flows as a function of the laser parameters. Counter-intuitively, the radius of the Marangoni region appears to remain constant and is not proportional to the thickness of the drop which decreases due to evaporation. We develop a model to predict the size of the Marangoni region that combines evaporative flows and laser-induced Marangoni flows. The experimental data are in good agreement with the predictions, allowing us to estimate the particle overconcentration factor resulting from the laser heating effects. The addition of surfactants to the solution allows the coupling of solutal Marangoni flows with thermal ones to achieve a final micron-scale deposit located at the laser spot. These results pave the way for new methods with high tunability provided by spatio-temporal light control for surface patterning applications.

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.

Photothermally Reconfigurable Colloidal Crystals at a Fluid Interface, a Generic Approach for Optically Tunable Lattice Properties

Optically addressable colloidal assembly at fluid interfaces is a highly desired component to generate reconfigurable 2D materials but has rarely been achieved and only with specific interface engineering. Here we describe a generic method to get optically reconfigurable colloidal crystals at the air/water interface and emphasize a new mechanism to convert light into tunable lattice properties. We use light-absorbing anionic particles adsorbed at the air/water interface in the presence of minute amounts of cationic surfactant, which self-assembled into closely packed polycrystalline structures by collectively deforming the surrounding interface. Low-intensity irradiation of these colloidal crystals results in unprecedented control of the interparticle spacing in a preserved crystalline state while, at a higher intensity, cycles of melting/recrystallization with a controllable transition kinetics can be achieved upon successive on/off stimulations. We show that this photoreversible melting originates from an initial thermocapillary stress, expanding the colloidal assembly against the local confinement, and an increase in particles diffusivity imposing the transition kinetics. With this mechanism, local irradiation leads to highly dynamic patterns, including self-healing or self-fed "living" crystals, while multiresponsive assembly is also achieved by controlling particle organization with both light and magnetic stimuli.

Applying droplets and films in evaporative lithography

This review covers experimental results of evaporative lithography and analyzes existing mathematical models of this method. Evaporating droplets and films are used in different fields, such as cooling of heated surfaces of electronic devices, diagnostics in health care, creation of transparent conductive coatings on flexible substrates, and surface patterning. A method called evaporative lithography emerged after the connection between the coffee ring effect taking place in drying colloidal droplets and naturally occurring inhomogeneous vapor flux densities from liquid-vapor interfaces was established. Essential control of the colloidal particle deposit patterns is achieved in this method by producing ambient conditions that induce a nonuniform evaporation profile from the colloidal liquid surface. Evaporative lithography is part of a wider field known as "evaporative-induced self-assembly" (EISA). EISA involves methods based on contact line processes, methods employing particle interaction effects, and evaporative lithography. As a rule, evaporative lithography is a flexible and single-stage process with such advantages as simplicity, low price, and the possibility of application to almost any substrate without pretreatment. Since there is no mechanical impact on the template in evaporative lithography, the template integrity is preserved in the process. The method is also useful for creating materials with localized functions, such as slipperiness and self-healing. For these reasons, evaporative lithography attracts increasing attention and has a number of noticeable achievements at present. We also analyze limitations of the approach and ways of its further development.

High-Rate Printing of Micro/Nanoscale Patterns Using Interfacial Convective Assembly

Printing of electronics has been receiving increasing attention from academia and industry over the recent years. However, commonly used printing techniques have limited resolution of micro- or sub-microscale. Here, a directed-assembly-based printing technique, interfacial convective assembly, is reported, which utilizes a substrate-heating-induced solutal Marangoni convective flow to drive particles toward patterned substrates and then uses van der Waals interactions as well as geometrical confinement to trap the particles in the pattern areas. The influence of various assembly parameters including type of mixing solvent, substrate temperature, particle concentration, and assembly time is investigated. The results show successful assembly of various nanoparticles in patterns of different shapes with a high resolution down to 25 nm. In addition, the assembly only takes a few minutes, which is two orders of magnitude faster than conventional convective assembly. Small-sized (diameter below 5 nm) nanoparticles tend to coalesce during the assembly process and form sintered structures. The fabricated silver nanorods show single-crystal structure with a low resistivity of 8.58 × 10^-5 Ω cm. With high versatility, high resolution, and high throughput, the interfacial convective assembly opens remarkable opportunities for printing next generation nanoelectronics and sensors.

Role of surfactant in controlling the deposition pattern of a particle-laden droplet: Fundamentals and strategies

Evaporation of particle-laden droplets has attracted wide attention propelled by the vast applications from disease diagnostics, bio-medicines, agriculture, inkjet printing to coating. Surfactant plays a vital role in controlling the deposition patterns of dried droplets, thanks to its extensive influences on particle transport through adsorbing at particle surface and droplet interfaces as well as suppressing or facilitating multiple flows. In order to accurately control the subtle morphology of a deposition, it is of significance to systematically elaborate the microscopic functions of surfactant, and bridge them to the various phenomena of a droplet. In this review, we first elucidate the effects of surfactant on the flow paradigms of capillary flow, solutal Marangoni flow, thermal Marangoni flow, and the mixed flow patterns as capillary force, thermal and solutal surface tensions are in competence or collaboration. Second, surfactant adsorption at particle surface and droplet interfaces modifying short-range and long-range forces such as electrostatic force, van der Waals force, capillary attraction, and hydrophobic bonding among particles and between particles and interfaces are introduced by the underlying mechanisms and approaches. Two phase diagrams are developed to respectively illustrate the roles of capillary force among particles, and the electrostatic interaction between particles and solid-liquid interface in modifying the deposition profiles. This review could build a fundamental framework of knowledge for evaporating particle-laden surfactant solution droplets, and may shed light on strategies to manipulate particle deposition in abundant fluidic-based techniques.

Photoswitchable Dissipative Two-Dimensional Colloidal Crystals

Control over particle interactions and organization at fluid interfaces is of great importance both for fundamental studies and practical applications. Rendering these systems stimulus-responsive is thus a desired challenge both for investigating dynamic phenomena and realizing reconfigurable materials. Here, we describe the first reversible photocontrol of two-dimensional colloidal crystallization at the air/water interface, where millimeter-sized assemblies of microparticles can be actuated through the dynamic adsorption/desorption behavior of a photosensitive surfactant added to the suspension. This allows us to dynamically switch the particle organization between a highly crystalline (under light) and a disordered (in the dark) phase with a fast response time (crystallization in ≈10 s, disassembly in ≈1 min). These results evidence a new kind of dissipative system where the crystalline state can be maintained only upon energy supply.