Dewetting patterns and molecular forces: a reconciliation

Instability of liquid Cu films on a SiO2 substrate

We study the instability of nanometric Cu thin films on SiO2 substrates. The metal is melted by means of laser pulses for some tens of nanoseconds, and during the liquid lifetime, the free surface destabilizes, leading to the formation of holes at first and then in later stages of the instability to metal drops on the substrate. By analyzing the Fourier transforms of the SEM (scanning electron microscope) images obtained at different stages of the metal film evolution, we determine the emerging length scales at relevant stages of the instability development. The results are then discussed within the framework of a long-wave model. We find that the results may differ whether early or final stages of the instability are considered. On the basis of the interpretation of the experimental results, we discuss the influence of the parameters describing the interaction of the liquid metal with the solid substrate. By considering both the dependence of dominant length scales on the film thickness and the measured contact angle, we isolate a model which predicts well the trends found in the experimental data.

Dynamic defrosting on nanostructured superhydrophobic surfaces

Water suspended on chilled superhydrophobic surfaces exhibits delayed freezing; however, the interdrop growth of frost through subcooled condensate forming on the surface seems unavoidable in humid environments. It is therefore of great practical importance to determine whether facile defrosting is possible on superhydrophobic surfaces. Here, we report that nanostructured superhydrophobic surfaces promote the growth of frost in a suspended Cassie state, enabling its dynamic removal upon partial melting at low tilt angles (<15°). The dynamic removal of the melting frost occurred in two stages: spontaneous dewetting followed by gravitational mobilization. This dynamic defrosting phenomenon is driven by the low contact angle hysteresis of the defrosted meltwater relative to frost on microstructured superhydrophobic surfaces, which forms in the impaled Wenzel state. Dynamic defrosting on nanostructured superhydrophobic surfaces minimizes the time, heat, and gravitational energy required to remove frost from the surface, and is of interest for a variety of systems in cold and humid environments.

Capillary fluctuations and film-height-dependent surface tension of an adsorbed liquid film

Our understanding of both structure and dynamics of adsorbed liquids heavily relies on the capillary wave Hamiltonian, but a thorough test of this model is still lacking. Here we study the capillary wave fluctuations of a liquid film with short-range forces adsorbed on a solid exhibiting van der Waals interactions. We show for the first time that the measured capillary wave spectrum right above the first order wetting transition provides an interface potential consistent with independent calculations from thermodynamic integration. However, the surface tension exhibits an oscillatory film thick dependence which reveals a hitherto unnoticed capillary wave broadening mechanism beyond mere interfacial displacements.

Impact of van der Waals interactions on single asperity friction

Single asperity measurements on Si wafers with variable SiO(2) layer thickness, yet identical roughness, revealed the influence of van der Waals (vdW) interactions on friction: on thin (1 nm) SiO(2) layers, higher friction and jump-off forces were observed as compared to thick (150 nm) SiO(2) layers. The vdW interactions were additionally controlled by a set of silanized Si wafers, exhibiting the same trend. The experimental results demonstrate the influence of the subsurface material and are quantitatively described by combining calculations of interactions of the involved materials and the Derjaguin-Müller-Toporov model.

Stability of thin liquid films and sessile droplets under confinement

The stability of nonvolatile thin liquid films and of sessile droplets is strongly affected by finite size effects. We analyze their stability within the framework of density functional theory using the sharp kink approximation, i.e., on the basis of an effective interface Hamiltonian. We show that finite size effects suppress spinodal dewetting of films because it is driven by a long-wavelength instability. Therefore nonvolatile films are stable if the substrate area is too small. Similarly, nonvolatile droplets connected to a wetting film become unstable if the substrate area is too large. This instability of a nonvolatile sessile droplet turns out to be equivalent to the instability of a volatile drop which can attain chemical equilibrium with its vapor.

Effect of Au nanoparticle spatial distribution on the stability of thin polymer films

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air-polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten.

Capillary wave confinement-induced stabilization of polymer films

We show that temporary confinement of polystyrene thin films by an elastomeric capping layer possessing nanoimprinted subcapillary wavelength (λ << λcap (20 μm)) line channels (amplitude A ≈ 120 nm) can suppress film dewetting on thermodynamically unfavorable substrates by arresting the amplitude growth and in-plane propagation of the destabilizing surface capillary waves. Confinement by either a smooth elastomer capping layer (A ≈ 1 nm) or with pattern features above the threshold dimension only retards dewetting but does not prevent it. The nanoimprint pattern is therefore essential to preventing dewetting, illustrating that only the penalty of elastomer deformation and interfacial tension reduction is insufficient.

Morphology Induced Spinodal Decomposition at the Surface of Symmetric Diblock Copolymer Films

Atomic force microscopy is used to study the ordering dynamics of symmetric diblock copolymer films. The films order to form a lamellar structure which results in a frustration when the film thickness is incommensurate with the lamellae. By probing the morphology of incommensurate films in the early ordering stages, we discover an intermediate phase of lamellae arranged perpendicular to the film surface. This morphology is accompanied by a continuous growth in amplitude of the film surface topography with a characteristic wavelength, indicative of a spinodal process. Using self-consistent field theory, we show that the observation of perpendicular lamellae suggests an intermediate state with parallel lamellae at the substrate and perpendicular lamellae at the free surface. The calculations confirm that the intermediate state is unstable to thickness fluctuations, thereby driving the spinodal growth of surface structures.

Influence of van der Waals interactions on morphology and dynamics in ultrathin liquid films at silicon oxide interfaces

Single molecule tracer diffusion studies of evaporating (thinning) ultrathin tetrakis-2-ethyl-hexoxysilane (TEHOS) films on silicon with 100 nm thermal oxide reveal a considerable slowdown of the molecular mobility within less than 4 nm above the substrate (corresponding to a few molecular TEHOS layers). This is related to restricted mobility and structure formation of the liquid in this region, in agreement with information obtained from a long-time ellipsometric study of thinning TEHOS films on silicon substrates with 100 nm thermal or 2 nm native oxide. Both show evidence for the formation of up to four layers. Additionally, on thermal oxide, a lateral flow of the liquid is observed, while the film on the native oxide forms an almost flat surface and shows negligible flow. Thus, on the 2 nm native oxide the liquid mobility is even more restricted in close vicinity to the substrate as compared to the 100 nm thermal oxide. In addition, we found a significantly smaller initial film thickness in case of the native oxide under similar dipcoating conditions. We ascribe these differences to van der Waals interactions with the underlying silicon in case of the native oxide, whereas the thermal oxide suffices to shield those interactions.

Structure Formation of Ultrathin PEO Films at Solid Interfaces&#8212;Complex Pattern Formation by Dewetting and Crystallization

The direct contact of ultrathin polymer films with a solid substrate may result in thin film rupture caused by dewetting. With crystallisable polymers such as polyethyleneoxide (PEO), molecular self-assembly into partial ordered lamella structures is studied as an additional source of pattern formation. Morphological features in ultrathin PEO films (thickness < 10 nm) result from an interplay between dewetting patterns and diffusion limited growth pattern of ordered lamella growing within the dewetting areas. Besides structure formation of hydrophilic PEO molecules, n-alkylterminated (hydrophobic) PEO oligomers are investigated with respect to self-organization in ultrathin films. Morphological features characteristic for pure PEO are not changed by the presence of the n-alkylgroups.

Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition

Surface energies are commonly used to determine the adhesion forces between materials. However, the component of surface energy derived from long-range forces, such as van der Waals forces, depends on the material's structure below the outermost atomic layers. Previous theoretical results and indirect experimental evidence suggest that the van der Waals energies of subsurface layers will influence interfacial adhesion forces. We discovered that nanometre-scale differences in the oxide layer thickness of silicon wafers result in significant macroscale differences in the adhesion of isolated gecko setal arrays. Si/SiO(2) bilayer materials exhibited stronger adhesion when the SiO(2) layer is thin (approx. 2 nm). To further explore how layered materials influence adhesion, we functionalized similar substrates with an octadecyltrichlorosilane monolayer and again identified a significant influence of the SiO(2) layer thickness on adhesion. Our theoretical calculations describe how variation in the SiO(2) layer thickness produces differences in the van der Waals interaction potential, and these differences are reflected in the adhesion mechanics. Setal arrays used as tribological probes provide the first empirical evidence that the 'subsurface energy' of inhomogeneous materials influences the macroscopic surface forces.

Effect of Long Range Interactions on the Glass Transition Temperature of Thin Polystyrene Films

The glass transition temperature (T_g) of thin polystyrene (PS) films supported on silicon wafers with oxide layers of varying thickness was characterized by the temperature dependence of the film thickness using ellipsometry. This allowed us to uncover how a long-range interaction affects the T_g of polymer films. As previously reported using a variety of methods, the T_g decreased with decreasing film thickness. However, the extent was not the same among the reports. In this study, we found that the T_g attenuation of a PS film of a given thickness was dependent on the oxide layer thickness of the silicon wafer via the long-range interaction.

Capillary wave dynamics of thin polymer films over submerged nanostructures

The surface dynamics of thin molten polystyrene films supported by nanoscale periodic silicon line-space gratings were investigated with x-ray photon correlation spectroscopy. Surface dynamics over these nanostructures exhibit high directional anisotropy above certain length scales, as compared to surface dynamics over flat substrates. A cutoff length scale in the dynamics perpendicular to the grooves is observed. This marks a transition from standard over-damped capillary wave behavior to suppressed dynamics due to substrate interactions.

Squeezing instabilities and delamination in elastic bilayers: a linear stability analysis

A linear stability analysis is presented to understand the instabilities that arise in an elastic bilayer, consisting of a very thin bottom layer (thickness < 100 nm) that acts as a wetting film and a top layer that acts as an adhesive film, when placed in contact proximity with an external rigid contactor. Depending on whichever layer is more compliant, "squeezing modes" of instability with a variety of length scales ranging from <<3h to <<3h (h: bilayer thickness) are found to be possible. The least length scales obtained are 0.1h. The squeezing instabilities are, however, accompanied by delamination of the film-film interface. The instability length scales, the strength of interactions required, and the delamination decrease as the compliance of the top film increases. Surface tension effects are found to have a stabilizing influence which increases the instability length scales and decreases the degree of delamination at the cost of high interaction penalty.

Numerical solutions of thin-film equations for polymer flows

We report on the numerical implementation of thin-film equations that describe the capillary-driven evolution of viscous films, in two-dimensional configurations. After recalling the general forms and features of these equations, we focus on two particular cases inspired by experiments: the leveling of a step at the free surface of a polymer film, and the leveling of a polymer droplet over an identical film. In each case, we first discuss the long-term self-similar regime reached by the numerical solution before comparing it to the experimental profile. The agreement between theory and experiment is excellent, thus providing a versatile probe for nanorheology of viscous liquids in thin-film geometries.

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials.

Ordered to isotropic morphology transition in pattern-directed dewetting of polymer thin films on substrates with different feature heights

Controlled dewetting of a thin polymer film on a topographically patterned substrate is an interesting approach for aligning isotropic dewetted structures. In this article, we investigate the influence of substrate feature height (H(S)) on the dewetting pathway and final pattern morphology by studying the dewetting of polystyrene (PS) thin films on grating substrates with identical periodicity (λ(P) = 1.5 μm), but H(S) varying between 10 nm and 120 nm. We identify four distinct categories of final dewetted morphology, with different extent of ordering: (1) array of aligned droplets (H(S) ≈ 120 nm); (2) aligned undulating ribbons (H(S) ≈ 70-100 nm); (3) multilength scale structures with coexisting large droplets uncorrelated to the substrate and smaller droplets/ribbons aligned along the stripes (H(S) ≈ 40-60 nm); and (4) large droplets completely uncorrelated to the substrate (H(S) < 25 nm). The distinct morphologies across the categories are attributed to two major factors: (a) whether the as-cast film is continuous (H(S)≤ 80 nm) or discontinuous (H(S)≥ 100 nm) and (b) in case of a continuous film, whether the film ruptures along each substrate stripe (H(S)≥ 70 nm) or with nucleation of random holes that are not correlated to the substrate features (H(S)≤ 60 nm). While the ranges of H(S) values indicated in the parentheses are valid for PS films with an equivalent thickness (h(E)) ≈ 50.3 nm on a flat substrate, a change in h(E) merely alters the cut-off values of H(S), as the final dewetted morphologies and transition across categories remain generically unaltered. We finally show that the structures obtained by dewetting on different H(S) substrates exhibits different levels of hydrophobicity because of combined spatial variation of chemical and topographic contrast along the surface. Thus, the work reported in this article can find potential application in fabricating surfaces with controlled wettability.

Stability of thin wetting films on chemically nanostructured surfaces

The morphology and stability of thin volatile wetting films on model chemically patterned surfaces composed of periodic arrays of alternating completely and partially wettable nanostripes are investigated. The equilibrium film morphology is recorded as a function of undersaturation using noncontact atomic force microscopy. Films spanning the entire pattern are found to be stable only for thicknesses in excess of a critical value, h(c), whereas thinner films spontaneously dewet the partially wettable regions of the substrate. The critical thickness h(c) increases linearly with the width of the partially wettable stripes in good agreement with an interface displacement model derived from microscopic density functional theory. These results provide detailed insights into the dewetting of thin films driven by competing intermolecular forces.

Competition between collapse and breakup in nanometer-sized thin rings using molecular dynamics and continuum modeling

We consider nanometer-sized fluid annuli (rings) deposited on a solid substrate and ask whether these rings break up into droplets due to the instability of Rayleigh-Plateau-type modified by the presence of the substrate, or collapse to a central drop due to the presence of azimuthal curvature. The analysis is carried out by a combination of atomistic molecular dynamics simulations and a continuum model based on a long-wave limit of Navier-Stokes equations. We find consistent results between the two approaches, and demonstrate characteristic dimension regimes which dictate the assembly dynamics.

Self-similarity and energy dissipation in stepped polymer films

The surface of a thin liquid film with a nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. We present the results of experiments on one of the simplest variable curvature surfaces: a thin polymer film with a step. Height profiles are measured as a function of time for a variety of molecular weights. The evolution of the profiles is shown to be self-similar. This self-similarity offers a precise measurement of the capillary velocity by comparison with numerical solutions of the thin film equation. We also derive a master expression for the time dependence of the excess free energy as a function of the material properties and film geometry. The experiment and theory are in excellent agreement and indicate the effectiveness of stepped polymer films to elucidate nanoscale rheological properties.

Selecting speed-dependent pathways for a programmable nanoscale texture by wet interfaces

The realization of well-defined and ordered structures on the nanoscale is a main issue in nanoscience and nanotechnology, biotechnology and other related fields like plastic or organic electronics. Among the bottom-up approaches, to date, self-assembly (equilibrium aggregates) received a major attention. In spite of this, far from equilibrium conditions allow for the generation of a wider landscape of organized systems depending on the set of control parameters employed. Under an adaptation vision of the structures, here we report some case studies showing how it is possible to programme and control the nanoscale features of ordered super- or supra-aggregates at wet interfaces by modulating the dynamic parameters. In particular, speed is foreseen as a threshold factor for changing the aggregation mechanism along with the shape and degree of order of the structures as well as, within a specific aggregation path, their size and defectivity.

Early and intermediate stages of guided dewetting in polystyrene thin films

We investigated the early and intermediate stages of the guided dewetting of polystyrene (PS) thin films on chemically patterned silicon, achieved by micro-contact printing of non-wettable self-assembling monolayers of an alkylsilane. Two different types of ordered patterns could be achieved depending on the annealing temperature and time. Study of the dynamics of hole growth revealed a deviation of the growth profile from the trend on homogeneous substrates, attributed to the pinning of the PS rims on the borders of the hydrophobic regions. The ordered patterns produced could be useful in applications that require spatially localized features of controlled surface chemistry, such as studies in proteomics, single cell studies, and biosensors.

High-speed in situ X-ray scattering of carbon nanotube film nucleation and self-organization

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.

Wetting on the microscale: shape of a liquid drop on a microstructured surface at different length scales

Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nano- and micrometer-sized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1-2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars' height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air-water interfaces.

Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity

The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading.

Influence of the subsurface composition of a material on the adhesion of staphylococci

Controlling the interface between bacteria and solid materials has become an important task in biomedical science. For a fundamental and comprehensive understanding of adhesion it is necessary to seek quantitative information about the involved interactions. Most studies concentrate on the modification of the surface (chemical composition, hydrophobicity, or topography) neglecting, however, the influence of the bulk material, which always contributes to the overall interaction via van der Waals forces. In this study, we applied AFM force spectroscopy and flow chamber experiments to probe the adhesion of Staphylococcus carnosus to a set of tailored Si wafers, allowing for a separation of short- and long-range forces. We provide experimental evidence that the subsurface composition of a substrate influences bacterial adhesion. A coarse estimation of the strength of the van der Waals forces via the involved Hamaker constants substantiates the experimental results. The results demonstrate that the uppermost layer is not solely responsible for the strength of adhesion. Rather, for all kinds of adhesion studies, it is equally important to consider the contribution of the subsurface.

On sub-T(g) dewetting of nanoconfined liquids and autophobic dewetting of crystallites

The glass transition temperature (T(g)) of thin films is reduced by nanoconfinement, but it is also influenced by the free surface and substrate interface. To gain more insights into their contributions, dewetting behaviors of n-pentane, 3-methylpentane, and toluene films are investigated on various substrates as functions of temperature and film thickness. It is found that monolayers of these molecules exhibit sub-T(g) dewetting on a perfluoro-alkyl modified Ni substrate, which is attributable to the evolution of a 2D liquid. The onset temperature of dewetting increases with film thickness because fluidity evolves via cooperative motion of many molecules; sub-T(g) dewetting is observed for films thinner than 5 monolayers. In contrast, monolayers wet substrates of graphite, silicon, and amorphous solid water until crystallization occurs. The crystallites exhibit autophobic dewetting on the substrate covered with a wetting monolayer. The presence of premelting layers is inferred from the fact that n-pentane crystallites disappear on amorphous solid water via intermixing. Thus, the properties of quasiliquid formed on the crystallite surface differ significantly from those of the 2D liquid formed before crystallization.

Wetting transparency of graphene

We report that graphene coatings do not significantly disrupt the intrinsic wetting behaviour of surfaces for which surface-water interactions are dominated by van der Waals forces. Our contact angle measurements indicate that a graphene monolayer is wetting-transparent to copper, gold or silicon, but not glass, for which the wettability is dominated by short-range chemical bonding. With increasing number of graphene layers, the contact angle of water on copper gradually transitions towards the bulk graphite value, which is reached for ~6 graphene layers. Molecular dynamics simulations and theoretical predictions confirm our measurements and indicate that graphene's wetting transparency is related to its extreme thinness. We also show a 30-40% increase in condensation heat transfer on copper, as a result of the ability of the graphene coating to suppress copper oxidation without disrupting the intrinsic wettability of the surface. Such an ability to independently tune the properties of surfaces without disrupting their wetting response could have important implications in the design of conducting, conformal and impermeable surface coatings.

Competition between dewetting and cross-linking in poly(N-vinylpyrrolidone)/polystyrene bilayer films

We investigated the dewetting of metastable poly(N-vinylpyrrolidone) (PNVP) thin films (45 nm) on top of polystyrene (PS) thin films (58 nm) as a function of annealing temperature and molecular weight of PS (96 and 6850 kg/mol). We focused on the competition between dewetting, occurring as a result of unfavorable intermolecular interactions at the PNVP/PS interface, and spontaneous cross-linking of PNVP, occurring during thermal annealing, as we recently reported (Telford, A. M.; James, M.; Meagher, L.; Neto, C. ACS Appl. Mater. Interfaces 2010, 2, 2399-2408). Using optical microscopy, we studied how the dewetting morphology and dynamics at different temperatures depended on the relative viscosity of the top PNVP film, which increased with cross-linking time, and of the bottom PS film. In the PNVP/PS96K system, cross-linking dominated over dewetting at temperatures below 180 °C, reducing drastically nucleated hole density and their maximum size, while above 180 °C the two processes reversed, with complete dewetting occurring at 200 °C. On the other hand, the PNVP/PS6850K system never achieved advanced dewetting stages as the dewetting was slower than cross-linking in the investigated temperature range. In both systems, dewetting of the PNVP films could be avoided altogether by thermally annealing the bilayers at temperatures where cross-linking dominated. The cross-linking was characterized quantitatively using neutron reflectometry, which indicated shrinkage and densification of the PNVP film, and qualitatively through selective removal of the bottom PS film. A simple model accounting for progressive cross-linking during the dewetting process predicted well the observed hole growth profiles and produced estimates of the PNVP cross-linking rate coefficients and of the activation energy of the process, in good agreement with literature values for similar systems.

NANOSCALE PATTERNING OF ORGANIC/INORGANIC MULTILAYERS BY SPINODAL DEFORMATION

We report on pattern formation which occurs spontaneously upon heating organic multilayer devices to the glass transition temperature of one component. The pattern formation is characterized as spinodal deformation process which is intrinsically possible in all devices with thin deformable layers in a thermodynamic non-equilibrium state. Size, morphology and direction of the pattern can be controlled which allows applications for nanostructuring optoelectronic devices.

Competing liquid phase instabilities during pulsed laser induced self-assembly of copper rings into ordered nanoparticle arrays on SiO2

Nanoscale copper rings of different radii, thicknesses, and widths were synthesized on silicon dioxide thin films and were subsequently liquefied via a nanosecond pulse laser treatment. During the nanoscale liquid lifetimes, the rings experience competing retraction dynamics and thin film and/or Rayleigh-Plateau types of instabilities, which lead to arrays of ordered nanodroplets. Surprisingly, the results are significantly different from those of similar experiments carried out on a Si surface. We use hydrodynamic simulations to elucidate how the different liquid/solid interactions control the different instability mechanisms in the present problem.

Modelling the evaporation of thin films of colloidal suspensions using dynamical density functional theory

Recent experiments have shown that various structures may be formed during the evaporative dewetting of thin films of colloidal suspensions. Nanoparticle deposits of strongly branched 'flower-like', labyrinthine and network structures are observed. They are caused by the different transport processes and the rich phase behaviour of the system. We develop a model for the system, based on a dynamical density functional theory, which reproduces these structures. The model is employed to determine the influences of the solvent evaporation and of the diffusion of the colloidal particles and of the liquid over the surface. Finally, we investigate the conditions needed for 'liquid-particle' phase separation to occur and discuss its effect on the self-organized nanostructures.

Coupling of microphase separation and dewetting in weakly segregated diblock co-polymer ultrathin films

We have studied the coupling behavior of microphase separation and autophobic dewetting in weakly segregated poly(ε-caprolactone)-block-poly(L-lactide) (PCL-b-PLLA) diblock co-polymer ultrathin films on carbon-coated mica substrates. At temperatures higher than the melting point of the PLLA block, the co-polymer forms a lamellar structure in bulk with a long period of L ∼ 20 nm, as determined using small-angle X-ray scattering. The relaxation procedure of ultrathin films with an initial film thickness of h = 10 nm during annealing has been followed by atomic force microscopy (AFM). In the experimental temperature range (100-140 °C), the co-polymer dewets to an ultrathin film of itself at about 5 nm because of the strong attraction of both blocks with the substrate. Moreover, the dewetting velocity increases with decreasing annealing temperatures. This novel dewetting kinetics can be explained by a competition effect of the composition fluctuation driven by the microphase separation with the dominated dewetting process during the early stage of the annealing process. While dewetting dominates the relaxation procedure and leads to the rupture of the ultrathin films, the composition fluctuation induced by the microphase separation attempts to stabilize them because of the matching of h to the long period (h ∼ 1/2L). The temperature dependence of these two processes leads to this novel relaxation kinetics of co-polymer thin films.

Wetting of a multiarm star-shaped molecule

The equilibrium contact angles and line tensions of macroscopic droplets, composed of star-shaped polystyrene (PS) macromolecules, on silicon oxide substrates, are shown to be smaller than their linear analogs, by up to approximately 1 and 2 orders of magnitude, respectively, depending on the size and functionality of the star-shaped molecule. A precursor layer, of lateral dimensions and of thicknesses on the order of nanometers, surrounds each droplet of low molecular weight linear PS chains. Droplets composed of star-shaped molecules possessing a sufficient number of arms, reside on a layer adsorbed to the substrate.

Electrostatic interaction forces in aqueous salt solutions of variable concentration and valency

We use atomic force microscopy (AFM) to determine electrostatic interactions between Si tips and Si wafers in aqueous electrolytes of variable composition. We demonstrate that dynamic force spectroscopy (DFS) in the frequency modulation (FM) mode with stiff cantilevers and sharp tips allows for a continuous detection of the tip-sample interactions without mechanical jump-to-contact instability and with substantially higher lateral resolution than standard colloidal probe measurements. For four different species of salt (NaCl, KCl, MgCl(2), CaCl(2)) we find repulsive electrostatic forces at the lowest salt concentrations (1 mM) that become progressively screened until they are dominated by attractive van der Waals forces at the highest concentration (100 mM). For the divalent cations the crossover from repulsive to attractive forces occurs at lower concentrations than for monovalent cations. Surface charges extracted from fits to standard Poisson-Boltzmann double layer theory indicate a rather weak dependence of the surface charge on the ion concentration. The high lateral resolution of our approach is illustrated by a 2D force field measurement over a patterned surface of a supported lipid bilayer on a mica substrate.

Formation of precise 2D Au particle arrays via thermally induced dewetting on pre-patterned substrates

The fabrication of precise 2D Au nanoparticle arrays over a large area is presented. The technique was based on pre-patterning of the substrate before the deposition of a thin Au film, and the creation of periodic particle arrays by subsequent dewetting induced by annealing. Two types of pre-patterned substrates were used: The first comprised an array of pyramidal pits and the second an array of circular holes. For the dewetting of Au films on the pyramidal pit substrate, the structural curvature-driven diffusion cooperates with capillarity-driven diffusion, resulting in the formation of precise 2D particle arrays for films within a structure dependent thickness-window. For the dewetting of Au films on the circular hole substrate, the periodic discontinuities in the films, induced by the deposition, can limit the diffusion paths and lead to the formation of one particle per individual separated region (holes or mesas between holes), and thus, result in the evolution of precise 2D particle arrays. The influence of the pre-patterned structures and the film thickness is analyzed and discussed. For both types of pre-patterned substrate, the Au film thickness had to be adjusted in a certain thickness-window in order to achieve the precise 2D particle arrays.

Self-assembly versus directed assembly of nanoparticles via pulsed laser induced dewetting of patterned metal films

A nanoscale, synthetic perturbation was all that was required to nudge a natural, self-assembly process toward significantly higher order. Metallic thin film strips were transformed into nanoparticle arrays by nanosecond, liquid-phase dewetting. Arrays formed according to an evolving Rayleigh-Plateau instability, yet nanoparticle diameter and pitch were poorly controlled. However, by patterning a nanoscale sinusoid onto the original strip edge, a precise nanoparticle diameter and pitch emerged superseding the naturally evolving Rayleigh-Plateau instability.