In situ observation of meniscus shape deformation with colloidal stripe pattern formation in convective self-assembly

Controlling Spatial Morphology of Microparticle Deposits via Thermocapillary Flows: Effect of Boundary Geometry

The production of particle deposits with a desired distribution geometry has significant potential for materials science, printing, and coating technologies. Most methods for achieving well-defined assemblies rely on the spontaneous evaporation of colloidal solutions on substrates with predetermined properties, or on precise control of particle arrangement by external stimuli. Here, we present a combined method that enables the production of centimeter-scale microparticle deposits with a desired geometric shape. The method is based on controlling the massive transport of microparticles by thermocapillary flow in a layer of volatile liquid in a cell with borders of the desired geometry. Capillary forces cause the liquid to be distributed in the cell, forming corner wetting menisci and the flat layer in the central area. The formation of particle deposits occurs in two stages, determined by the flow regime. At the initial stage, the axisymmetric thermocapillary flow occurs in the flat part of the layer, resulting in the circular shape of the particle deposit. During the transition to the second stage of assembling thermocapillary flow is localized in the corner wetting menisci that results in reshaping the current particle deposit to match the geometry of the cell borders. Here, we demonstrated the creation of circular, square, and triangular shapes of the patterns of polystyrene microparticles using a point heater located at the geometric center of the cell. The proposed method is reliable, easy to implement, and potentially capable of producing a wide variety of deposit geometries, making it an attractive technique for patterning and modifying surface properties with particles of any type.

Nanoparticle Assembly: From Self-Organization to Controlled Micropatterning for Enhanced Functionalities

Nanoparticles form long-range micropatterns via self-assembly or directed self-assembly with superior mechanical, electrical, optical, magnetic, chemical, and other functional properties for broad applications, such as structural supports, thermal exchangers, optoelectronics, microelectronics, and robotics. The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. This article provides a comprehensive review of nanoparticle assembly formed primarily via the balance of forces at the nanoscale (e.g., van der Waals, colloidal, capillary, convection, and chemical forces) and nanoparticle-template interactions (e.g., physical confinement, chemical functionalization, additive layer-upon-layer). The review commences with a general overview of nanoparticle self-assembly, with the state-of-the-art literature review and motivation. It subsequently reviews the recent progress in nanoparticle assembly without the presence of surface templates. Manufacturing techniques for surface template fabrication and their influence on nanoparticle assembly efficiency and effectiveness are then explored. The primary focus is the spatial organization and orientational preference of nanoparticles on non-templated and pre-templated surfaces in a controlled manner. Moreover, the article discusses broad applications of micropatterned surfaces, encompassing various fields. Finally, the review concludes with a summary of manufacturing methods, their limitations, and future trends in nanoparticle assembly.

Optimization of Colloidal Gold Nanoparticles on Porous Anodic Aluminum Oxide Substrates for Refractometric Sensing

A new composite metal-insulator-metal (MIM) system consisting of exceptionally dense non-close-packed (NCP) arrays of gold or silver nanoparticles, porous anodic aluminum oxide (PAAO), and bulk aluminum substrate interacts strongly with visible light and may become a very useful component for optical applications. The proposed MIM structure can be synthesized using accessible lithography-free chemical and physical processes (anodization and capillary force assisted colloidal particle deposition) that are suitable for the low-cost production of specialized devices. Here, we present a systematic study to determine the essential MIM structure parameters (nanoparticle size and PAAO layer thickness) for localized surface plasmon resonance (LSPR) refractometric sensing. A performance comparison was done by recording the spectra of scattered light upon angled illumination in media with different refractive indices. A clear advantage for maximizing the signal to background ratio was observed in the case of 60 and 80 nm Au nanoparticles with a PAAO thickness in a narrow range between 300 and 375 nm. Sensitivity exceeding a 200 nm peak wavelength shift per refractive index unit was found for 60 nm Au nanoparticles on approximately 500-nm-thick PAAO. The experimental observations were supported by finite-difference time-domain (FDTD) simulations.

Highly Ordered Inverse Opal Structures Synthesized from Shape-Controlled Nanocrystal Building Blocks

Three-dimensional ordered porous materials known as inverse opal films (IOFs) were synthesized using nanocrystals with precisely defined morphologies. Comprehensive theoretical and experimental studies of the volume fraction ratio and electrostatic interactions between nanocrystals and polystyrene templating particles enabled the formation of highly ordered crack-free photonic structures. The synthetic strategy was first demonstrated using titanium dioxide (TiO₂ ) nanocrystals of different shapes and then generalized to assemble nanocrystals of other functional materials, such as indium tin oxide and zinc-doped ferrite. Tunable photocatalytic activity of the TiO₂ IOFs, modulated through the choice of the shape of TiO₂ nanocrystals in conjunction with selecting desired macroscopic features of the IOF, was further explored. In particular, enhanced activity is observed for crack-free, highly ordered IOFs whose photonic properties can improve light absorption via the slow light effect. This study opens new opportunities in designing multi-length-scale porous nanoarchitectures having enhanced performance in a variety of applications.

Homogenization of initial cell distribution by secondary flow of medium improves cell culture efficiency

Homogenization of the initial cell distribution is essential for effective cell development. However, there are few previous reports on efficient cell seeding methods, even though the initial cell distribution has a large effect on cell proliferation. Dense cell regions have an inverse impact on cell development, known as contact inhibition. In this study, we developed a method to homogenize the cell seeding density using secondary flow, or Ekman transportation, induced by orbital movement of the culture dish. We developed an orbital shaker device that can stir the medium in a 35-mm culture dish by shaking the dish along a circular orbit with 2 mm of eccentricity. The distribution of cells in the culture dish can be controlled by the rotational speed of the orbital shaker, enabling dispersion of the initial cell distribution. The experimental results indicated that the cell density became most homogeneous at 61 rpm. We further evaluated the cell proliferation after homogenization of the initial cell density at 61 rpm. The results revealed 36% higher proliferation for the stirred samples compared with the non-stirred control samples. The present findings indicate that homogenization of the initial cell density by Ekman transportation contributes to the achievement of higher cell proliferation.

Drying Droplets with Soluble Surfactants

We propose a theory for the drying of liquid droplets of surfactant solutions. We show that the added surfactant hinders droplet receding and facilitates droplet spreading, causing a complex behavior of the contact line of an evaporating droplet: the contact line first recedes, then advances, and finally recedes again. We also show that the surfactant can change the deposition pattern from mountain-like to volcano-like and then to coffee-ring-like. Specially, when the contact line motion undergoes a clear receding-advancing transition, a two-ring pattern is formed. The mechanism of the two-ring formation is different from the stick-slip mechanism proposed previously and may be tested experimentally.

The effect of particle wettability on the stick-slip motion of the contact line

Contact line dynamics is crucial in determining the deposition patterns of evaporating colloidal droplets. Using high-speed interferometry, we directly observe the stick-slip motion of the contact line in situ and are able to resolve the instantaneous shape of the inkjet-printed, evaporating pico-liter drops containing nanoparticles of varying wettability. Integrated with post-mortem optical profilometry of the deposition patterns, the instantaneous particle volume fraction and hence the particle deposition rate can be determined. The results show that the stick-slip motion of the contact line is a strong function of the particle wettability. While the stick-slip motion is observed for nanoparticles that are less hydrophilic (i.e., particle contact angle θ ≈ 74° at the water-air interface), which results in a multiring deposition, a continuous receding of the contact line is observed for more hydrophilic nanoparticles (i.e., θ ≈ 34°), which leaves a single-ring pattern. A model is developed to predict the number of particles required to pin the contact line based on the force balance of the hydrodynamic drag, interparticle interactions, and surface tension acting on the particles near the contact line with varying particle wettability. A three-fold increase in the number of particles required for pinning is predicted when the particle wettability increases from the wetting angle of θ ≈ 74° to θ ≈ 34°. This finding explains why particles with greater wettability form a single-ring pattern and those with lower wettability form a multi-ring pattern. In addition, the particle deposition rate is found to depend on the particle wettability and vary with time.

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

We evaporate aqueous suspensions in a microchamber to explore the connection between the morphology of the nanoparticle deposits at nanometer resolutions and at micrometer and hundreds of micrometers resolutions. Repulsive or weakly attractive electrical double-layer and van der Waals surface forces render the deposition of detached particles and small aggregates at nanometer resolutions. However, strongly attractive surface forces render the dense deposition of large aggregates. At greater length resolutions, the deposit morphology is further governed by evaporation-mediated transport of particles in the volatile suspension. We use experiment and theory to show that the contributions of the different mechanisms to the deposit morphology are mediated by particle coagulation and by particle adsorption to the substrate. The nanometer deposit morphology and particle transport render the morphology of the deposits at greater length resolutions, where it may take the shape of crude or smooth particulate micropatterns or continuous particulate coating layers.

Role of Vapor Mass Transfer in Flow Coating of Colloidal Dispersions in the Evaporative Regime

In flow-coating processes at low substrate velocity, solvent evaporation occurs during the film withdrawal and the coating process directly yields a dry deposit. In this regime, often referred to as the evaporative regime, several works performed on blade-coating-like configurations have reported a deposit thickness h_d proportional to the inverse of the substrate velocity V. Such a scaling can be easily derived from simple mass conservation laws, assuming that evaporation occurs on a constant distance, referred to as the evaporation length, noted L_ev in the present paper and of the order of the meniscus size. However, the case of colloidal dispersions deserves further attention. Indeed, the coating flow leads to a wet film of densely packed colloids before the formation of the dry deposit. This specific feature is related to the porous nature of the dry deposit, which can thus remain wet when capillary forces are strong enough to prevent the receding of the solvent through the pores of the film (the so-called pore-emptying). The length of this wet film may possibly be much larger than the meniscus size, therefore modifying the solvent evaporation rate, as well as the scaling h_d ∼ 1/V. This result was suggested recently by different groups using basic modeling and assuming for simplicity a uniform evaporation rate over the wet film. In this article, we go a step further and investigate the effect of multidimensional vapor mass transfer in the gas phase on L_ev and h_d in the specific case of colloidal dispersions. Using simplified models, we first provide analytical expressions in asymptotic cases corresponding to 1D or 2D diffusive vapor transport. These theoretical investigations then led us to show that L_ev is independent of the evaporation rate amplitude, and roughly independent of its spatial distribution. Conversely, h_d strongly depends on the characteristics of vapor mass transfer in the gas phase, and different scaling laws are obtained for the 1D or the 2D case. These theoretical findings are finally tested by comparison with experimental results supporting our theoretical simplified approach.

Multiscale Modeling of the Three-Dimensional Meniscus Shape of a Wetting Liquid Film on Micro-/Nanostructured Surfaces

The design of structured surfaces for increasing the heat flux dissipated during boiling and evaporation processes via enhanced liquid rewetting requires prediction of the liquid meniscus shape on these surfaces. In this study, a general continuum model is developed to predict the three-dimensional meniscus shape of liquid films on micro/nanostructured surfaces based on a minimization of the system free energy that includes solid-liquid van der Waals interaction energy, surface energy, and gravitational potential. The continuum model is validated at the nanoscale against molecular dynamics simulations of water films on gold surfaces with pyramidal indentations, and against experimental measurements of water films on silicon V-groove channels at the microscale. The validated model is used to investigate the effect of film thickness and surface structure depth on the meniscus shape. The meniscus is shown to become more conformal with the surface structure as the film thickness decreases and the structure depth increases. Assuming small interface slope and small variation in film thickness, the continuum model can be linearized to obtain an explicit expression for the meniscus shape. The error of this linearized model is quantitatively assessed and shown to increase with increasing structure depth and decreasing structure pitch. The model developed can be used for accurate prediction of three-dimensional meniscus shape on structured surfaces with micro/nano-scale features, which is necessary for determining the liquid delivery rate and heat flux dissipated during thin-film evaporation. The linearized model is useful for rapid prediction of meniscus shape when the structure depth is smaller than or comparable to the liquid film thickness.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

Contact Line Dynamics during the Evaporation of Extended Colloidal Thin Films: Influence of Liquid Polarity and Particle Size

Exercising control over the evaporation of colloidal suspensions is pivotal to modulate the coating characteristics for specific uses, wherein the interactions among the liquid, the particles, and the substrate control the process. In the present study, the contact line dynamics of a receding colloidal liquid film consisting of particles of distinctly different sizes (nominal diameters 0.055 and 1 μm and surface unmodified) during evaporation is analyzed. The role of the liquid polarity is also investigated by replacing the polar liquid (water) with a relatively nonpolar liquid (isopropyl alcohol) in the colloidal suspension. The characteristics of the evaporating receding meniscus, namely, the film thickness and the curvature are experimentally evaluated using an image-analyzing interferometry technique. The experimental results are assessed in conjunction with the augmented Young-Laplace equation, highlighting the roles of the relevant components of the disjoining pressure and the polarity of the liquid involved in the colloidal suspension.

Flexible, Transparent, Thickness-Controllable SWCNT/PEDOT:PSS Hybrid Films Based on Coffee-Ring Lithography for Functional Noncontact Sensing Device

Flexible transparent conductive films (FTCFs) as the essential components of the next generation of functional circuits and devices are presently attracting more attention. Here, a new strategy has been demonstrated to fabricate thickness-controllable FTCFs through coffee ring lithography (CRL) of single-wall carbon nanotube (SWCNT)/poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (

PEDOT

PSS) hybrid ink. The influence of ink concentration and volume on the thickness and size of hybrid film has been investigated systematically. Results show that the final FTCFs present a high performance, including a homogeneous thickness of 60-65 nm, a sheet resistance of 1.8 kohm/sq, a visible/infrared-range transmittance (79%, PET = 90%), and a dynamic mechanical property (>1000 cycle, much better than ITO film), respectively, when SWCNT concentration is 0.2 mg/mL, ink volume is 0.4 μL, drying at room temperature. Moreover, the benefits of these kinds of FTCFs have been verified through a full transparent, flexible noncontact sensing panel (3 × 4 sensing pixels) and a flexible battery-free wireless sensor based on a humidity sensing mechanism, showing excellent human/machine interaction with high sensitivity, good stability, and fast response/recovery ability.