Signatures of van der Waals and Electrostatic Forces in the Deposition of Nanoparticle Assemblies

Depletion-Induced Self-Assembly of Colloidal Particles on a Solid Substrate

We investigate the depletion contributions to the self-assembly of microcolloids on solid substrates. The assembly is driven by the exclusion of nanoparticles and nonadsorbing polymers from the depletion zone between the microcolloids in the liquid and the underlying substrate. The model system consists of 1 μm polystyrene particles that we deposit on a flat glass slab in an electrolyte solution. Using polystyrene nanoparticles and poly(acrylic acid) polymers as depleting agents, we demonstrate in our experiments that nanoparticle concentrations of 0.5% (w/v) support well-ordered packing of microcolloids on glass, while the presence of polymers leads to irregular aggregate deposition structures. A mixture of nanoparticles and polymers enhances the formation of colloidal aggregate and particulate surface coverage compared to using the polymers alone as a depletion agent. Moreover, tuning the polymer ionization state from pH 4 to 9 modifies the polymer conformational state and radius of gyration, which in turn alters the microcolloid deposition from compact multilayers to flocculated structures. Our study provides entropic strategies for manipulating particulate assembly on substrates from dispersed to continuous coatings.

Particle Network Self-Assembly of Similar Size Sub-Micron Calcium Alginate and Polystyrene Particles Atop Glass

Particle-mediated self-assembly, such as nanocomposites, microstructure formation in materials, and core-shell coating of biological particles, offers precise control over the properties of biological materials for applications in drug delivery, tissue engineering, and biosensing. The assembly of similar-sized calcium alginate (CAG) and polystyrene sub-micron particles is studied in an aqueous sodium nitrate solution as a model for particle-mediated self-assembly of biological and synthetic mixed particle species. The objective is to reinforce biological matrices by incorporating synthetic particles to form hybrid particulate networks with tailored properties. By varying the ionic strength of the suspension, the authors alter the energy barriers for particle attachment to each other and to a glass substrate that result from colloidal surface forces. The particles do not show monotonic adsorption trend to glass with ionic strength. Hence, apart from DLVO theory-van der Waals and electrostatic interactions-the authors further consider solvation and bridging interactions in the analysis of the particulate adsorption-coagulation system. CAG particles, which support lower energy barriers to attachment relative to their counterpart polystyrene particles, accumulate as dense aggregates on the glass substrate. Polystyrene particles adsorb simultaneously as detached particles. At high electrolyte concentrations, where electrostatic repulsion is largely screened, the mixture of particles covers most of the glass substrate; the CAG particles form a continuous network throughout the glass substrate with pockets of polystyrene particles. The particulate structure is correlated with the adjustable energy barriers for particle attachment in the suspension.

Advancements in Nanoparticle Deposition Techniques for Diverse Substrates: A Review

Nanoparticle deposition on various substrates has gained significant attention due to the potential applications of nanoparticles in various fields. This review paper comprehensively analyzes different nanoparticle deposition techniques on ceramic, polymeric, and metallic substrates. The deposition techniques covered include electron gun evaporation, physical vapor deposition, plasma enriched chemical vapor deposition (PECVD), electrochemical deposition, chemical vapor deposition, electrophoretic deposition, laser metal deposition, and atomic layer deposition (ALD), thermophoretic deposition, supercritical deposition, spin coating, and dip coating. Additionally, the sustainability aspects of these deposition techniques are discussed, along with their potential applications in anti-icing, antibacterial power, and filtration systems. Finally, the review explores the importance of deposition purities in achieving optimal nanomaterial performance. This comprehensive review aims to provide valuable insights into state-of-the-art techniques and applications in the field of nanomaterial deposition.

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.

Progress in Studies of Surface Nanotextures and Coatings with Nanomaterials on Glass for Anti-Dust Functionality

Dust pollution presents a wide range of adverse effects to product functionalities and the quality of human life. For instance, when dust particles deposit on solar photovoltaic panels, sunlight absorption is significantly reduced, and solar-to-electrical energy conversion yield may be lowered by 51%- Conventional (manual) dust removal methods are costly, consume significant material resources, and cause irreparable damage to the solar glass surface. Therefore, it is critical to develop glass surfaces that can clean themselves or are easily cleaned by natural forces. Many approaches have been attempted to reduce dust deposition, such as developing superhydrophobic surfaces and preparing anti-static surfaces. This paper reviews the recent progress in studies of anti-dust and cleaning mechanisms or methodologies, which include investigation into micro- and nano-sized dust properties, dust deposition processes and adhesion mechanisms to surfaces, and the state-of-the-art approaches to anti-dust and easy-cleaning functions that tailor surface micro-/nanotextures, lowering surface energy via nanocoatings, and enhancing anti-static properties with nanomaterials. We compare the advantages and disadvantages of various approaches and discuss the research prospects. We envision that future research will be focused on developing transparent surfaces with multiple dust-proof functions to cope with dust-burdening operating environments.

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.

Soft Bioelectronics Based on Nanomaterials

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

Revisiting the Electroacoustic Phenomenon in the Presence of Surface Acoustic Waves

Recently, one has been observing abundant studies on the application of surface acoustic waves (SAWs) in solid substrates for manipulating liquids and particulates in micron-to-nanometer thick films and channels and in porous media. At these length scales, contributions of SAWs to the electrical double layer (EDL) of ions and of the latter to particulates and flow may become appreciable. However, the nature of the interplay between SAWs and EDLs is unknown. We demonstrate the contribution of a SAW to the near-equilibrium electrical and ion-concentration fields in an EDL near inert and piezoelectric substrates. In particular, we concentrate on the leakage of transient and steady components of electrical potential through the excited EDL. Far from the solid, the leakage may be interpreted by different models of the EDL to give information about the EDL characteristic relaxation time, ζ-potential, and the Stern layer therein. In addition, the analysis given here may explain observed SAW-induced electrochemical effects on piezoelectric substrates.

Plasmonic nanorod array for effective photothermal therapy in hyperthermia

Optical properties of anisotropic gold nanorod arrays inside anodic aluminium oxide substrates enhance the longitudinal absorption intensities and the hyperthermia cancer cell killing at 42.1 °C under photothermal laser exposures at 671 nm.

Self-assembly in biobased nanocomposites for multifunctionality and improved performance

Concerns of petroleum dependence and environmental pollution prompt an urgent need for new sustainable approaches in developing polymeric products. Biobased polymers provide a potential solution, and biobased nanocomposites further enhance the performance and functionality of biobased polymers. Here we summarize the unique challenges and review recent progress in this field with an emphasis on self-assembly of inorganic nanoparticles. The conventional wisdom is to fully disperse nanoparticles in the polymer matrix to optimize the performance. However, self-assembly of the nanoparticles into clusters, networks, and layered structures provides an opportunity to address performance challenges and create new functionality in biobased polymers. We introduce basic assembly principles through both blending and in situ synthesis, and identify key technologies that benefit from the nanoparticle assembly in the polymer matrix. The fundamental forces and biobased polymer conformations are discussed in detail to correlate the nanoscale interactions and morphology with the macroscale properties. Different types of nanoparticles, their assembly structures and corresponding applications are surveyed. Through this review we hope to inspire the community to consider utilizing self-assembly to elevate functionality and performance of biobased materials. Development in this area sets the foundation for a new era of designing sustainable polymers in many applications including packaging, construction chemicals, adhesives, foams, coatings, personal care products, and advanced manufacturing.

Self-Assembly of Nanoparticles from Evaporating Sessile Droplets: Fresh Look into the Role of Particle/Substrate Interaction

We studied the dependence of solid deposit shape obtained by free drying of sessile drops on particle concentration and Derjaguin-Landau-Verwey-Overbeek (DLVO) particle/substrate interaction. In contrast to previous contributions using pH as a control parameter of interactions, we investigated an unprecedentedly wide range of concentrations and particle/substrate DLVO forces by modifying the nature of the substrate and particles as well as their size and surface chemistry, whereas long-distance repulsive interactions between particles were maintained for most of the drying time. Our main result is that the different shapes of deposits obtained by modifying the particle concentration are the same in the different regimes of concentration regardless of particle/substrate interaction in the studied range of DLVO forces and particle concentrations. The second result is that, contrary to expectations, the dominant morphology of dry patterns at low particle concentration always shows a dotlike pattern for all the studied systems.

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.

In situ structure and force characterization of 2D nano-colloids at the air/water interface

The control of self-assembly and the related interactions among nanoparticles (NPs) at liquid surfaces and interfaces represents a stimulating experimental challenge to fully understand the behaviour of nano-colloids confined in a 2D asymmetric environment, in turn prompting the building of novel NP-based functional monolayers. Here, we first investigate the structural evolution of a model mixed surfactant/NP monolayer as a function of the surfactant/NP bulk ratio finding that, at ratios lower than 20, the adsorption at the air/water interface of surfactant-decorated NPs is dominant. We then employed these 2D nano-colloidal monolayers as model systems for grazing incidence small angle X-ray scattering measurements, performed using synchrotron radiation, while compressing the monolayers in a Langmuir trough. The simultaneous determination of the compression work and the related reduction of the inter-particle distance at the interface enabled, for the first time, the quantitative characterization of the forces acting between adsorbed NPs, as well as their dispersion law with the inter-particle distance. Distinct surfactant reorganization processes are proposed to interpret the measured forces and the characteristic inter-particle distances.

Nanoscale Topographical Fluctuations: A Key Factor for Evaporative Colloidal Self-Assembly

This work investigates the role of surface parameters such as the nanoscale roughness, topography, and skewness of smooth and rough Si surfaces in the shape of patterns left by evaporating colloidal droplets of spherical polystyrene particles. The droplet contact angle, colloidal deposition pattern, crack density, and rim growth velocities are experimentally evaluated for varying roughness. The contact angle and rim growth rate are found to be more for rough surfaces in comparison to smooth ones. Roughness also helps in reducing stress in the drying droplets, thereby impeding the process of crack formation as exemplified by the experimental results. The altered Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions emerging from the contribution of nanoscale roughness are theoretically evaluated for each differently rough substrate-particle combination. The forces have been calculated by considering large- and small-scale roughness parameters of the experimental surfaces. The experimental findings have been duly corroborated by theoretical estimates. Finally, it is observed that the skewness of the surface and the small-scale asperity radius bear a correlation with the DLVO forces and subsequently with the ring deposit pattern. The present understanding of the influence of surface fluctuations on evaporative self-assembly would enable one to choose the right topographic surface for particular applications.