Spontaneous pattern formation by dip coating of colloidal suspensions on homogeneous surfaces

Dip coating of shear-thinning particulate suspensions

Dip coating a planar substrate with a suspension of particles in a shear-thinning liquid will entrain particles in the liquid film, facilitating filtration and sorting of particles. Experiments were performed for both monodisperse and bidisperse particle suspensions of shear-thinning Xanthan Gum solutions. Particle entrainment occurs when the coating thickness at the stagnation point of the thin film flow is larger than the particle diameter. A model is developed to predict the entrainment criteria using lubrication theory applied to an Ostwald power-law fluid which yields a modified Landau-Levich-Derjaguin (LLD) law governing the coating film thickness that depends upon a properly defined capillary number Ca. The critical withdrawal velocity for particle entrainment depends upon the particle size and fluid rheology through a relationship between Ca and the bond number Bo, which agrees well with our model predictions and prior experimental results of A. Sauret et al. Phys. Rev. Fluids, 2019, 4, 054303 for the limiting case of Newtonian suspensions. Single particle entrainment and particle clustering is observed for monodisperse suspensions, which depends on Ca and the particle volume fraction ϕ. In bidisperse suspensions, particle sorting can occur whereby only the smaller particles are entrained in the film over an active filtration range of Ca and Bo, which also agrees well with our model predictions.

Continuous protein-density gradients: A new approach to correlate physical cues with cell response

To assess cellular behavior within heterogeneous tissues, such as bone, skin, and nerves, scaffolds with biophysical gradients are required to adequately replicate the in vivo interaction between cells and their native microenvironment. In this study, we introduce a strategy for depositing ultrathin films comprised of laminin-111 with precisely controlled biophysical gradients onto planar substrates using the Langmuir-Blodgett (LB) technique. The gradient is created by controlled desynchronization of the barrier compression and substrate withdrawal speed during the LB deposition process. Characterization of the films was performed using techniques such as atomic force microscopy and confocal fluorescence microscopy, enabling the comprehensive analysis of biophysical parameters along the gradient direction. Furthermore, human adipose-derived stem cells were seeded onto the gradient films to investigate the influence of protein density on cell attachment, showing that the distribution of the cells can be modulated by the arrangement of the laminin at the air-water interface. The presented approach not only allowed us to gain insights into the intricate interplay between biophysical cues and cell behavior within complex tissue environments, but it is also suited as a screening approach to determine optimal protein concentrations to achieve a target cellular output.

A Novel Approach for Colloidal Lithography: From Dry Particle Assembly to High-Throughput Nanofabrication

We introduce a novel approach for colloidal lithography based on the dry particle assembly into a dense monolayer on an elastomer, followed by mechanical transfer to a substrate of any material and curvature. This method can be implemented either manually or automatically and it produces large area patterns with the quality obtained by the state-of-the-art colloidal lithography at a very high throughput. We first demonstrated the fabrication of nanopatterns with a periodicity ranging between 200 nm and 2 μm. We then demonstrated two nanotechnological applications of this approach. The first one is antireflective structures, fabricated on silicon and sapphire, with different geometries including arrays of bumps and holes and adjusted for different spectral ranges. The second one is smart 3D nanostructures for mechanostimulation of T cells that are used for their effective proliferation, with potential application in cancer immunotherapy. This new approach unleashes the potential of bottom-up nanofabrication and paves the way for nanoscale devices and systems in numerous applications.

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.

Diphenylalanine Deposition by Dip-Coating from Acidic Solutions: Fibers and Closed Homogeneous Films

A simple but precisely controllable strategy by molecular assembly that enables the construction of biomaterials is always in the development. Dip-coating deposition of diphenylalanine (FF) onto planar solid substrates from aqueous acidic (acetic, propanoic, formic, and HCl) solutions is studied as a function of the process control parameters (deposition speed, initial concentration of FF and acids, and external gas flow). The results are studied by optical microscopy, AFM, and ellipsometry. For low acidity and low FF concentrations, FF forms microfibers, nanofibers, or stripes of fiber aggregates. For higher acidity and FF concentrations, closed films of FF of remarkably smooth surfaces are found. The thickness of these films can be well-controlled by the FF concentration and the deposition speed and explained by the evaporation regime. These unusual results provide new possibilities to fabricate more abundant structures by a simple strategy and develop a candidate for biological membrane areas.

Nanosphere Lithography: A Versatile Approach to Develop Transparent Conductive Films for Optoelectronic Applications

Transparent conductive films (TCFs) are irreplaceable components in most optoelectronic applications such as solar cells, organic light-emitting diodes, sensors, smart windows, and bioelectronics. The shortcomings of existing traditional transparent conductors demand the development of new material systems that are both transparent and electrically conductive, with variable functionality to meet the requirements of new generation optoelectronic devices. In this respect, TCFs with periodic or irregular nanomesh structures have recently emerged as promising candidates, which possess superior mechanical properties in comparison with conventional metal oxide TCFs. Among the methods for nanomesh TCFs fabrication, nanosphere lithography (NSL) has proven to be a versatile platform, with which a wide range of morphologically distinct nanomesh TCFs have been demonstrated. These materials are not only functionally diverse, but also have advantages in terms of device compatibility. This review provides a comprehensive description of the NSL process and its most relevant derivatives to fabricate nanomesh TCFs. The structure-property relationships of these materials are elaborated and an overview of their application in different technologies across disciplines related to optoelectronics is given. It is concluded with a perspective on current shortcomings and future directions to further advance the field.

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.

Dip coating of cylinders with Newtonian fluids

HYPOTHESIS

The Landau-Levich-Derjaguin (LLD) theory is widely applied to predict the film thickness in the dip-coating process. However, the theory was designed only for flat plates and thin fibers. Fifty years ago, White and Tallmadge attempted to generalize the LLD theory to thick rods using a numerical solution for a static meniscus and the LLD theory to forcedly match their numeric solution with the LLD asymptotics. The White-Talmadge solution has been criticized for not being rigorous yet widely used in engineering applications mostly owing to the lack of alternative solutions. A new set of experiments significantly expanding the range of White-Tallmadge conditions showed that their theory cannot explain the experimental results. We then hypothesized that the results of LLD theory can be improved by restoring the non-linear meniscus curvature in the equation. With this modification, the obtained equation should be able to describe static menisci on any cylindrical rods and the film profiles observed at non-zero rod velocity.

EXPERIMENT

To test the hypothesis, we distinguished capillary forces from viscous forces by running experiments with different rods and at different withdrawal velocities and video tracking the menisci profiles and measuring the weight of deposited films. The values of film thickness were then fitted with a mathematical model based on the modified LLD equation. We also fitted the meniscus profiles.

FINDINGS

The results show that the derived equation allows one to reproduce the results of the LLD theory and go far beyond those to include rods of different radii. A new set of experimental data together with the White-Tallmadge experimental data are explained with the modified LLD theory. A set of simple formulas approximating numeric results have been derived. These formulas can be used in engineering applications for the prediction of the coating thickness.

Enhanced Ordering of Block Copolymer Thin Films upon Addition of Magnetic Nanoparticles

The influence of magnetite nanoparticles coated with poly(acrylic acid) (Fe₃O₄@PAA NPs) on the organization of block copolymer thin films via a self-assembly process was investigated. Polystyrene-b-poly(4-vinylpyridine) films were obtained by the dip-coating method and thoroughly examined by X-ray reflectivity, transmission electron microscopy, atomic force microscopy, and grazing incidence small-angle scattering. Magnetic properties of the films were probed via superconducting quantum interference device (SQUID) magnetometry. It was demonstrated that due to the hydrogen bonding between P4VP and PAA, the Fe₃O₄@PAA NPs segregate selectively inside P4VP domains, enhancing the microphase separation process. This in turn, together with employing carefully optimized dip-coating parameters, results in the formation of hybrid thin films with highly ordered nanostructures. The addition of Fe₃O₄@PAA nanoparticles does not change the average interdomain spacing in the film lateral nanostructure. Moreover, it was shown that the nanoparticles can easily be removed to obtain well-ordered nanoporous templates.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Large-scale patterning of π-conjugated materials by meniscus guided coating methods

Printed organic electronics has attracted considerable interest in recent years as it enables the fabrication of large-scale, low-cost electronic devices, and thus offers significant possibilities in terms of developing new applications in various fields. Easy processing is a prerequisite for the development of low-cost, flexible and printed plastics electronics. Among processing techniques, meniscus guided coating methods are considered simple, efficient, and low-cost methods to fabricate electronic devices in industry. One of the major challenges is the control of thin film morphology, molecular orientations and directional alignment of polymer films during coating processes. Herein, the recent progress of emerging field of meniscus guided printing organic semiconductor materials is discussed. The first part of this report briefly summarizes recent advances in meniscus guided coating techniques. The second part discusses periodic deposits and patterned deposition at moving contact lines, where the mass-transport influences film morphology due to convection at the triple contact line. The last section summarizes our strategy to fabricate large-scale patterning of π-conjugated polymers using meniscus guided method.

Digital Assembly of Colloidal Particles for Nanoscale Manufacturing

From unravelling the most fundamental phenomena to enabling applications that impact our everyday lives, the nanoscale world holds great promise for science, technology and medicine. However, the extent of its practical realization would rely on manufacturing at the nanoscale. Among the various nanomanufacturing approaches being investigated, the bottom-up approach involving assembly of colloidal nanoparticles as building blocks is promising. Compared to a top-down lithographic approach, particle assembly exhibits advantages such as smaller feature size, finer control of chemical composition, less defects, lower material wastage, and higher scalability. The capability to assemble colloidal particles one by one or "digitally" has been heavily sought as it mimics the natural way of making matter and enables construction of nanomaterials with sophisticated architectures. This progress report provides an insight into the tools and techniques for digital assembly of particles, including their working mechanisms and demonstrated particle assemblies. Examples of nanomaterials and nanodevices are presented to demonstrate the strength of digital assembly in nanomanufacturing.

Dip-coating of suspensions

Withdrawing a plate from a suspension leads to the entrainment of a coating layer of fluid and particles on the solid surface. In this article, we study the Landau-Levich problem in the case of a suspension of non-Brownian particles at moderate volume fraction 10% < φ < 41%. We observe different regimes depending on the withdrawal velocity U, the volume fraction of the suspension φ, and the diameter of the particles 2a. Our results exhibit three coating regimes. (i) At small enough capillary number Ca, no particles are entrained, and only a liquid film coats the plate. (ii) At large capillary number, we observe that the thickness of the entrained film of suspension is captured by the Landau-Levich law using the effective viscosity of the suspension η(φ). (iii) At intermediate capillary numbers, the situation becomes more complicated with a heterogeneous coating on the substrate. We rationalize our experimental findings by providing the domain of existence of these three regimes as a function of the fluid and particles properties.

The meniscus on the outside of a circular cylinder: From microscopic to macroscopic scales

We systematically study the meniscus on the outside of a small circular cylinder vertically immersed in a liquid bath in a cylindrical container that is coaxial with the cylinder. The cylinder has a radius R much smaller than the capillary length, κ^-1, and the container radius, L, is varied from a small value comparable to R to ∞. In the limit of L≪κ^-1, we analytically solve the general Young-Laplace equation governing the meniscus profile and show that the meniscus height, Δh, scales approximately with Rln(L/R). In the opposite limit where L≫κ^-1,Δh becomes independent of L and scales with Rln(κ^-1/R). We implement a numerical scheme to solve the general Young-Laplace equation for an arbitrary L and demonstrate the crossover of the meniscus profile between these two limits. The crossover region has been determined to be roughly 0.4κ^-1≲L≲4κ^-1. An approximate analytical expression has been found for Δh, enabling its accurate prediction at any values of L that ranges from microscopic to macroscopic scales.

Dopamine-assisted co-deposition: An emerging and promising strategy for surface modification

Mussel-inspired chemistry based on polydopamine (PDA) deposition has been developed as a facile and universal method for the surface modification of various materials. However, the inherent shortcomings of PDA coatings still impede their practical applications in the development of functional materials. In this review, we introduce the recent progress in the emerging dopamine-assisted co-deposition as a one-step strategy for functionalizing PDA-based coatings, and improving them in the aspects of deposition rate, morphology uniformity, surface wettability and chemical stability. The co-deposition mechanisms are categorized and discussed according to the interactions of dopamine or PDA with the introduced co-component. We also emphasize the influence of these interactions on the properties of the resultant PDA-based coatings. Meanwhile, we conclude the representative potential applications of those dopamine-assisted co-deposited coatings in material science, especially including separation membranes and biomaterials. Finally, some important issues and perspectives for theoretical study and applications are briefly discussed.

Scalable Directed Assembly of Highly Crystalline 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) Films

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution-based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we show scalable and controllable directed assembly of highly crystalline 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) films via a dip-coating process. Self-aligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters: the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions. A field-effect hole mobility up to 3.99 cm² V^-1 s^-1 is obtained. Owing to the highly scalable crystalline film formation, the dip-coating directed assembly process could be a great candidate for manufacturing next-generation electronics. Meanwhile, the film formation mechanism discussed in this paper could provide a general guideline to prepare other organic semiconducting films from small molecule solutions.

Surfactants as mesogenic agents in layer-by-layer assembled polyelectrolyte/surfactant multilayers: nanoarchitectured "soft" thin films displaying a tailored mesostructure

Interfacial supramolecular architectures displaying mesoscale organized components are of fundamental importance for developing materials with novel or optimized properties. Nevertheless, engineering the multilayer assembly of different building blocks onto a surface and exerting control over the internal mesostructure of the resulting film is still a challenging task in materials science. In the present work we demonstrate that the integration of surfactants (as mesogenic agents) into layer-by-layer (LbL) assembled polyelectrolyte multilayers offers a straightforward approach to control the internal film organization at the mesoscale level. The mesostructure of films constituted of hexadecyltrimethylammonium bromide, CTAB, and polyacrylic acid, PAA (of different molecular weights), was characterized as a function of the number of assembled layers. Structural characterization of the multilayered films by grazing-incidence small-angle X-ray scattering (GISAXS), showed the formation of mesostructured composite polyelectrolyte assemblies. Interestingly, the (PAA/CTA)n assemblies prepared with low PAA molecular weight presented different mesostructural regimes which were dependent on the number of assembled layers: a lamellar mesophase for the first bilayers, and a hexagonal circular mesophase for n ≥ 7. This interesting observation was explained in terms of the strong interaction between the substrate and the first layers leading to a particular mesophase. As the film increases its thickness, the prevalence of this strong interaction decreases and the supramolecular architecture exhibits a "bulk" mesophase. Finally, we demonstrated that the molecular weight of the polyelectrolyte has a considerable impact on the meso-organization for the (PAA/CTA)n assemblies. We consider that these studies open a path to new rational methodologies to construct "nanoarchitectured" polyelectrolyte multilayers.

Large-Area Self-Assembly of Silica Microspheres/Nanospheres by Temperature-Assisted Dip-Coating

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO₂) microspheres/nanospheres (SPs) as monolayers over large areas (∼cm²). The area over which self-assembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (T_s), which allows precise control over the meniscus shape. Furthermore, the formation of periodic stripes of SAMs, with excellent dimensional control (stripe width and stripe-to-stripe spacing), is demonstrated using a suitable set of dip-coating parameters. These findings establish the role of T_s, and other parameters such as withdrawal speed (V_w), withdrawal angle (θ_w), and withdrawal step length (L_w). For T_s ranged between 25 and 80 °C, the morphological analysis of dip-coatings shows layered structures comprising of defective layers (25-60 °C), single layers (70 °C), and multilayers (>70 °C) owing to the variation of SP flux at the meniscus/substrate assembling interface. At T_s = 70 °C, there is an optimum V_w, approximately equal to the downshift speed of the meniscus (V_m = 1.3 μm/s), which allows the SAM formation over areas (2.25 cm²) roughly 10 times larger than reported in the literature using nanospheres. Finally, the large-area SAM is used to demonstrate the enhanced performance of antireflective coatings for photovoltaic cells and to create metal nanomesh for Si nanowire synthesis.

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.

Nanostructure nitride light emitting diodes via the Talbot effect using improved colloidal photolithography

We report here a new approach of Talbot effect based colloidal photolithography for fabricating nanostructure light emitting diodes (LEDs). By employing a rigid metal nanohole array template (RDT) as the diffraction grating and a polysiloxane-based spin on dielectric (SOD) as the thickness-controllable spacer layer, various InGaN/GaN nanostructure LEDs have been fabricated. Three-dimensional finite-difference time-domain (3D-FDTD) simulations have been conducted to verify the proposed approach. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) have been performed to investigate the optical properties of the obtained nanostructure LEDs. Our report shows significance in fabricating two dimensional (2D) functional nanostructures and understanding the optical properties of various nanostructure InGaN/GaN LEDs.

From evaporation-induced self-assembly to shear-induced alignment

The functionality of compact nanostructured thin films depends critically on the degree of order and hence on the underlying ordering mechanisms during film formation. For dip coating of rigid nanorods the counteracting mechanisms, evaporation-induced self-assembly (EISA) and shear-induced alignment (SIA) have recently been identified as competing ordering mechanisms. Here, we show how to achieve highly ordered and homogeneous thin films by controlling EISA and SIA in dip coating. Therefore we identify the influences of the process parameters including temperature, initial volume fraction and nanorod aspect ratio on evaporation-induced convective flow and externally applied shear forces and evaluate the resulting films. The impact of evaporation and shear can be distinguished by analysing film thickness, surface order and bulk order by careful in situ SAXS, Raman and SEM-based image analysis. For the first time we derive processing guidelines for the controlled application of EISA and SIA towards highly ordered thin nematic films.

Self-Assembly of Single-Sized and Binary Colloidal Particles at Air/Water Interface by Surface Confinement and Water Discharge

We present an innovative apparatus allowing self-assembly at air/water interface in a smooth and reproducible way. The combination of water discharge and surface confinement of the area over which self-assembly takes place allows transfer of the assembled monolayer without any risk of damage to the colloidal crystal. As we demonstrate, the designed approach offers remarkable advantages in terms of cost and robustness compared to state-of-the art methods and is suitable for the fabrication of highly ordered monolayers even for more challenging assembly experiments such as transfer on rough substrates or assembly of binary colloids. Hence, our apparatus represents a significant headway toward high scale production of large area colloidal crystals. For the binary colloid assembly experiments, we also report the first experimental demonstration of a morphology based on the alternation of three and four small particles in the interstices between large particles.

Polarized Raman scattering and SEM combined full characterization of self-assembled nematic thin films

Elongated particles are predestined for a fast transfer of optical and electronical signals in a preferred direction, which is mandatory for a quick response in optoelectronic devices. The performance of the material is based on the quality of defect less alignment of the particles. On this account we present an easy non-invasive methodology for characterization of both surface and bulk order. The characterization of bulk order is performed by orientation dependent variation of the polarized Raman scattering signal on large areas by mapping. Scanning electron microscopy and image analysis on the surface complete the characterization. New insights in dip coated nematic structures clearly show the interplay of evaporation induced and shear-induced self-assembly and reveal a comprehensive mechanistic picture for nanorod assembly: the shear force dominated regime orients the particle in direction of withdrawal. At low withdrawal velocity, however, shear forces and evaporation counteract to produce a three-layered film where the top and bottom layers are oriented perpendicular to each other. The middle layer gives a clear evidence for a reorientation by convective flow.

Hydrodynamic confinement and capillary alignment of gold nanorods

Controlling the alignment and orientation of nanorods on various surfaces poses major challenges. In this work, we investigate hydrodynamic confinement and capillary alignment of gold nanorod assembly on chemically stripe-patterned substrates. The surface patterns consist of alternating hydrophilic and hydrophobic micrometer wide stripes; a macroscopic wettability gradient enables controlling the dynamics of deposited suspension droplets. We show that drying of residual liquid on the hydrophilic stripes gives rise to spatially localized deposition and alignment of the nanorods. Moreover, a universal relation between the extent of order within the single layers of nanoparticles and the lateral dimension of the deposits is presented and discussed.

Flow-induced alignment of (100) fcc thin film colloidal crystals

The realization of structural diversity in colloidal crystals obtained by self-assembly techniques remains constrained by thermodynamic considerations and current limits on our ability to alter structure over large scales using imposed fields and confinement. In this work, a convective-based procedure to fabricate multi-layer colloidal crystal films with extensive square-like symmetry is enabled by periodic substrate motion imposed during the continuous assembly. The formation of film-spanning domains of (100) fcc symmetry as a result of added vibration is robust across a range of micron-scale monosized spherical colloidal suspensions (e.g., polystyrene, silica) as well as substrate surface chemistries (e.g., hydrophobic, hydrophilic). The generation of extensive single crystalline (100) fcc domains as large as 15 mm(2) and covering nearly 40% of the colloidal crystalline film is possible by simply tuning coating conditions and multi-layer film thickness. Preferential orientation of the square-packed domains with respect to the direction of deposition is attributed to domain generation based upon a shear-related mechanism. Visualization during assembly gives clues toward the mechanism of this flow-driven self-assembly method.

High-power passively mode-locked Nd:YVO(4) laser using SWCNT saturable absorber fabricated by dip coating method

Passive mode locked laser is typically achieved by the Semiconductor Saturable absorber Mirror, SESAM, saturable absorber, which is produced by expensive and complicated metal organic chemical vapor deposition method. Carbon based single wall carbon nanotube (SWCNT), saturable absorber, is a promising material which is capable to produce stable passive mode-locking in the high power laser cavity over a wide operational wavelength range. This study has successfully demonstrated the high power mode locking laser system operating at 1 micron by using SWCNT based absorbers fabricated by dip coating method. The proposed fabrication method is practical, simple and cost effective for fabricating SWCNT saturable absorber. The demonstrated high power Nd:YVO(4) mode-locked laser operating at 1064nm have maximum output power up to 2.7W,with the 167MHz repetition rate and 3.1 ps pulse duration, respectively. The calculated output pulse energy and peak power are 16.1nJ and 5.2kW, respectively.

Nanocomposited silicone hydrogels with a laser-assisted surface modification for inhibiting the growth of bacterial biofilm

Compared to the commercial silicone catheters, the nanocomposited silicone hydrogel with a laser-assisted surface modification can reduce the growth of bacteria from 1.20 × 10⁶ CFU cm⁻² to 3.69 × 10⁵ CFU cm⁻², almost an order of magnitude.

Transition from stripe-like patterns to a particulate film using driven evaporating menisci

Better control of colloidal assembly by convective deposition is particularly helpful in particle templating. However, knowledge of the different factors that can alter colloidal patterning mechanisms is still insufficient. Deposit morphology is strongly ruled by contact line dynamics, but the wettability properties of the substrate can alter it drastically. In this work, we experimentally examined the roles of substrate contact angle hysteresis and receding contact angle using driven evaporating menisci similar to the dip-coating technique but at a low capillary number. We used smooth substrates with very different wettability properties and nanoparticles of different sizes. For fixed withdrawal velocity, evaporation conditions, and nanoparticle concentration, we analyzed the morphology of the deposits formed on each substrate. A gradual transition from stripe-like patterns to a film was observed as the contact angle hysteresis and receding contact angle were lowered.

Two-dimensional patterns of poly(N-isopropylacrylamide) microgels to spatially control fibroblast adhesion and temperature-responsive detachment

Thermoresponsive poly(N-isopropyl acrylamide) (PNIPAM) microgels were patterned on polystyrene substrates via dip coating, creating cytocompatible substrates that provided spatial control over cell adhesion. This simple dip-coating method, which exploits variable substrate withdrawal speeds forming particle suspension stripes of densely packed PNIPAM microgels, while spacings between the stripes contained sparsely distributed PNIPAM microgels. The assembly of three different PNIPAM microgel patterns, namely, patterns composed of 50 μm stripe/50 μm spacing, 50 μm stripe/100 μm spacing, and 100 μm stripe/100 μm spacing, was verified using high-resolution optical micrographs and ImageJ analysis. PNIPAM microgels existed as monolayers within stripes and spacings, as revealed by atomic force microscopy (AFM). Upon cell seeding on PNIPAM micropatterned substrates, NIH3T3 fibroblast cells preferentially adhered within spacings to form cell patterns. Three days after cell seeding, cells proliferated to form confluent cell layers. The thermoresponsiveness of the underlying PNIPAM microgels was then utilized to recover fibroblast cell sheets from substrates simply by lowering the temperature without disrupting the underlying PNIPAM microgel patterns. Harvested cell sheets similar to these have been used for multiple tissue engineering applications. Also, this simple, low-cost, template-free dip-coating technique can be utilized to micropattern multifunctional PNIPAM microgels, generating complex stimuli-responsive substrates to study cell-material interactions and allow drug delivery to cells in a spatially and temporally controlled manner.

Hydrodynamically driven colloidal assembly in dip coating

We study the hydrodynamics of dip coating from a suspension and report a mechanism for colloidal assembly and pattern formation on smooth substrates. Below a critical withdrawal speed where the coating film is thinner than the particle diameter, capillary forces induced by deformation of the free surface prevent the convective transport of single particles through the meniscus beneath the film. Capillary-induced forces are balanced by hydrodynamic drag only after a minimum number of particles assemble within the meniscus. The particle assembly can thus enter the thin film where it moves at nearly the withdrawal speed and rapidly separates from the next assembly. The interplay between hydrodynamic and capillary forces produces periodic and regular structures below a critical ratio Ca(2/3)/sqrt[Bo] < 0.7, where Ca and Bo are the capillary and Bond numbers, respectively. An analytical model and numerical simulations are presented for the case of two-dimensional flow with circular particles in suspension. The hydrodynamically driven assembly documented here is consistent with stripe pattern formations observed experimentally in dip coating.

High-mobility ZnO nanorod field-effect transistors by self-alignment and electrolyte-gating

High mobility, solution-processed field-effect transistors are important building blocks for flexible electronics. Here we demonstrate the alignment of semiconducting, colloidal ZnO nanorods by a simple solvent evaporation technique and achieve high electron mobilities in field-effect transistors at low operating voltages by electrolyte-gating with ionic liquids. The degree of alignment varies with nanorod length, concentration and solvent evaporation rate. We find a strong dependence of electron mobility on the degree of alignment but less on the length of the nanorods. Maximum field-effect mobilities reach up to 9 cm(2) V(-1) s(-1) for optimal alignment. Because of the low process temperature (150 °C), ZnO nanorod thin films are suitable for application on flexible polymer substrates.

Stuck in traffic: Patterns of powder adhesion

The adhesion of fine particles to surfaces is important for applications ranging from drug delivery to fouling of solar cells. In this letter, we show that powder adhesion can occur in unexpected patterns, concentrating particular grain types in some locations and clearing them from others, and we propose a straightforward traffic model that appears to reproduce many of the behaviors seen. The model predicts different patterns depending on inter-particle cohesion, and we find in both experiment and model that adhesion occurs in three distinct stages.

Morphological changes during annealing of polyethylene nanocrystals

Polymer crystals are metastable and exhibit morphological changes when being annealed. To observe morphological changes on molecular scales we started from small nanometer-sized crystals of highly folded long-chain polymers. Micron-sized stripes consisting of monolayers or stacks of several layers of flat-on oriented polyethylene nanocrystals were generated via evaporative dewetting from an aqueous dispersion. We followed the morphological changes in time and at progressively higher annealing temperatures by determining the topography and viscoelastic properties of such assemblies of nanocrystals using atomic force microscopy. Due to smallness and high surface-to-volume ratio of the nanocrystals, already at 75 °C, i.e. about 60 degrees below the nominal melting point, the lateral size of the crystal coarsened. Intriguingly, this occurred without a noticeable reduction in the number of folds per polymer chain. Starting at around 110 °C, chain folds were progressively removed leading to crystal thickening. At higher temperatures, but still below the melting point, prolonged annealing allowed for surface diffusion of molten polymers on the initially bare substrate, leading eventually to the disappearance of crystals. We compared these results to the behavior of the same nanocrystals annealed in an aqueous dispersion and to bulk samples.