Dewetting patterns and molecular forces: a reconciliation

Motion of nanodroplets near chemical heterogeneities

We investigate the dynamics of nanoscale droplets in the vicinity of chemical steps which separate parts of a substrate with different wettabilities. Due to long-ranged dispersion forces, nanodroplets positioned on one side of the step perceive the different character of the other side even at a finite distance from the step, leading to a dynamic response. The direction of the ensuing motion of such droplets depends not only on the difference between the equilibrium contact angles on these two parts but in particular on the difference between the corresponding Hamaker constants. Therefore, the motion is not necessarily directed toward the more wettable side and can also be different from that of droplets which span the step.

Coarsening of unstable thin films subject to gravity

Thin films of viscous fluids coating hydrophobic substrates are unstable to dewetting instabilities, and long-time evolution leads to the formation of an array of near-equilibrium droplets connected by ultrathin fluid layers. In the absence of gravity, previous use of lubrication theory has shown that coarsening dynamics will ensue-the system will evolve by successively eliminating small drops to yield fewer larger drops. While gravity has only a weak influence on the initial thin film, we show that it has a significant influence on the later stages of the coarsening dynamics, dramatically slowing the rate of coarsening for large drops. Small drops are relatively unaffected, but as coarsening progresses, these aggregate into larger drops whose shape and dynamics are dominated by gravity. The change in the mean drop shape causes a corresponding gradual transition from power-law coarsening to a logarithmic behavior.

Direct observation of nanoparticle embedding into the surface of a polymer melt

Direct embedding of metal nanoparticles (NPs) into the surface of a polymer melt is observed by TEM and a new embedding mechanism proposed. Upon annealing above the glass transition temperature of polystyrene (PS), NPs (20 nm gold) are rapidly covered by a thin PS wetting layer, h* approximately 1.3-1.8 nm (i.e., about two or three monomers). Because it creates capillary pressure on a NP, this "universal" wetting layer is proposed to be responsible for NP embedding. The value of h* is independent of the molecular weight of PS and constant during the embedding process. The value of h* is found to be similar to the equilibrium wetting layer thickness of a polymer melt spreading on a metal substrate. Using a model that includes the spreading coefficient, long-range van der Waals interactions, and a chain-stretching penalty, h* is shown to be independent of the molecular weight of the polymer. Using this model and the measured value of h*, the interfacial energy between Au NP and PS is estimated to be 8.7 J/m2.

Periodic organic nanodot patterns for optical memory

The fabrication of organic nanodots from photoswitchable fulgide molecules is shown. The dots are formed by dewetting of thin precursor films of the organic molecules on topographically structured substrates. In this way, we are able to control size, density, and arrangement of nanodots on millimeter-squared sized areas. We show that nanodots can be switched between isomeric molecular conformation reversibly.

Slippage of Newtonian liquids: influence on the dynamics of dewetting thin films

Slippage of Newtonian liquids in the presence of a solid substrate is a newly found phenomenon, the origin of which is still under debate. In this article, we present a new analysis method to extract the slip length. Enhancing the slip of liquids is an important issue for microfluidic devices that demand for high throughput at low pumping power. We study the velocity of short-chained liquid polystyrene (PS) films dewetting from nonwettable solid substrates. We show how the dynamics of dewetting is influenced by slippage, and we compare the results of two types of substrates that give rise to different slip lengths. As substrates, Si wafers that have been coated with octadecyltrichlorosilane (OTS) or dodecyltrichlorosilane (DTS) were used. Our results demonstrate that the dewetting velocity for PS films on DTS is significantly larger than on OTS and that this difference originates from the different slip lengths of the liquid on top of the two surfaces. For PS films of thickness between 130 and 230 nm, we find slip lengths between 400 nm and 6 microm, depending on substrate and temperature.

Apparent dewetting of ultrathin multilayered polyelectrolyte films incubated in aqueous environments

We have investigated and characterized changes in film morphology and surface structure that occur when ultrathin multilayered polyelectrolyte films fabricated from linear poly(ethylene imine) (LPEI), sodium poly(styrene sulfonate) (SPS), and two hydrolytically degradable polyamines (polymers 1 and 2) are incubated in physiologically relevant environments. Characterization of the physical erosion profiles of films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 (approximately 80 nm thick) by atomic force microscopy (AFM), reflective optical microscopy, and scanning electron microscopy (SEM) demonstrated that these materials undergo large-scale changes in surface structure and morphology upon incubation in phosphate-buffered saline (PBS) at 37 degrees C. The patterns and structures generated during this transformation (e.g., nucleation and growth of holes, coalescence of holes, formation of cell-type structures, and the subsequent breakup of these features into droplets) are similar in many ways to those observed for the dewetting of thin films of conventional polymers, such as polystyrene, on nonwetting surfaces. The processes reported here are sufficiently slow (they occur over approximately 100 h) and occur under sufficiently mild conditions (e.g., incubation in PBS at 37 degrees C) to permit characterization and quantification of the structures and features that arise during the course of these transformations. The apparent dewetting of these ultrathin films upon exposure to aqueous environments creates future opportunities to investigate and characterize processes of mass transport in this class of ionically cross-linked assemblies.

Quantifying hydrodynamic slip: a comprehensive analysis of dewetting profiles

To characterize nontrivial boundary conditions of a liquid flowing past a solid, the slip length is commonly used as a measure. From the profile of a retracting liquid front (e.g., measured with atomic force microscopy), the slip length can be extracted with the help of a Stokes model for a thin liquid film dewetting from a solid substrate. Specifically, we use a lubrication model derived from the Stokes model for strong slippage and linearize the film profile around the flat, unperturbed film. For small slip lengths, we expand the linearized full Stokes model for small slopes up to third order. Using the respective model, we obtain, in addition to the slip length, the capillary number, from which we can estimate the viscosity of the fluid film. We compare numerical and experimental results, test the consistency and the validity of the models/approximations, and give an easy-to-follow guide of how they can be used to analyze experiments.

Controlling pattern formation in nanoparticle assemblies via directed solvent dewetting

We have achieved highly localized control of pattern formation in two-dimensional nanoparticle assemblies by direct modification of solvent dewetting dynamics. A striking dependence of nanoparticle organization on the size of atomic force microscope-generated surface heterogeneities is observed and reproduced in numerical simulations. Nanoscale features induce a rupture of the solvent-nanoparticle film, causing the local flow of solvent to carry nanoparticles into confinement. Microscale heterogeneities instead slow the evaporation of the solvent, producing a remarkably abrupt interface between different nanoparticle patterns.

Thermal noise influences fluid flow in thin films during spinodal dewetting

Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Because of thermal fluctuations the spectrum changes from an exponential to a power law decay for large wave vectors. Also the time evolution of the typical wave vector of unstable perturbations exhibits noise-induced coarsening that is absent in deterministic hydrodynamic flow.

Theory of slope-dependent disjoining pressure with application to Lennard–Jones liquid films

A liquid film of thickness h<100 nm is subject to additional intermolecular forces, which are collectively called disjoining pressure Pi. Since Pi dominates at small film thicknesses, it determines the stability and wettability of thin films. Current theory derived for uniform films gives Pi=Pi(h). This solution has been applied recently to non-uniform films and becomes unbounded near a contact line as h-->0. Consequently, many different effects have been considered to eliminate or circumvent this singularity. We present a mean-field theory of Pi that depends on the slope h(x) as well as the height h of the film. When this theory is implemented for Lennard-Jones liquid films, the new Pi=Pi(h,h(x)) is bounded near a contact line as h-->0. Thus, the singularity in Pi(h) is artificial because it results from extending a theory beyond its range of validity. We also show that the new Pi can capture all three regimes of drop behavior (complete wetting, partial wetting, and pseudo-partial wetting) without altering the signs of the long and short-range interactions. We find that a drop with a precursor film is linearly stable.

Partially wetting thin liquid films: structure and dynamics studied with coherent x rays

We studied the surface structure of thin liquid films vapor deposited on solid substrates in a partial wetting situation by means of coherent x-ray scattering. No dynamics has been observed showing the absence of capillary waves on liquid films partially wetting a substrate. Instead an exponential form of the height-height correlation function has been found pointing toward a solidlike behavior of the thin liquid films at large length scales. The exact surface structure and degree of replication with the substrate depend on the deposition rate of the molecules.

Nonsolvent-induced dewetting of thin polymer films

The process of nonsolvent-induced dewetting of thin polystyrene (PS) films on hydrophilic surfaces at room temperature has been studied by using water as a nonsolvent. It is observed that the process of nonsolvent-induced dewetting is greatly different from other previous dewetting processes. The PS film is found in nonviscous state in our study. A mechanism of nonsolvent-induced dewetting is deduced in an order of penetration, replacement, and coalescent, and it is different from other previous dewetting mechanisms. The results of experiments are analyzed from thermodynamics and dynamics to support the hypothetical mechanism.

Substrate wetting and topographically induced ordering of amorphous PI-b-PFS block-copolymer domains

The substrate wetting of an amorphous, low-glass-transition-temperature spherical poly(isoprene-block-ferrocenylsilane) (PI-b-PFS) block copolymer and the alignment of the microdomains in grooves of various geometry are studied. Compositional analysis by time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS) indicates the presence of both PI and PFS directly at the film-substrate interface on silicon and silica substrates. The TOF-SIMS depth-profiling study indicates a transition in the packing of the domains between the two-dimensional (2D) monolayer and 3D, thicker layers. In a monolayer of domains, a hexagonal packing is adopted. In films of two or three layers, the hexagonal packing reorganizes towards a body-centered cubic (bcc) packing by the extension of the copolymer chains in the direction normal to the substrate, as indicated by an increase in spacing between PFS layers and an increase in domain size. For thicker layers, a bcc morphology with the (110) plane parallel to the substrate is found to extend from the free surface downwards. Films of one monolayer of domains of the copolymer exhibit long-range lateral ordering on the micrometer scale on flat substrates without high-temperature annealing. On topographically patterned silicon substrates the position of the domains of the minority PFS phase directly near the side walls is fixed by the neutral wetting condition. Successful positioning of the block-copolymer spheres in linear and hexagonal grooves is achieved in grooves up to 1.3 microm wide, whereby the hexagonal grooves demonstrate complete 2D alignment. In circular pits, this graphoepitaxial effect is absent.

Dewetting and surface properties of ultrathin films of cellulose esters

Surface properties of ultrathin films of cellulose esters deposited onto silicon wafers have been investigated by means of contact angle measurements and atomic force microscopy (AFM). Cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) films adsorbed or spin-coated onto Si wafers were annealed up to one week. Film stability was monitored by AFM. Dewetting has been observed for CA and CAP. Only CAB films with lower degree of esterification presented dewetting, CAB films with high degree of butyrate were stable even after one week annealing. Surface energy of CA, CAP, and CAB was indirectly determined by contact angle measurements using drops of water, formamide and diiodomethane. The surface energy decreased as the size of alkyl ester group or the degree of esterification increased because van der Waals interactions became weaker. Effective Hamaker constant A(eff) was calculated for CA, CAP, and CAB onto Si wafers in air. Negative values of A(eff) were found for CA, CAP, and lower butyrate content CAB, which are related to instability and agree with dewetting phenomena observed by AFM. In contrast, a positive A(eff) was determined for higher butyrate content CAB, corroborating with experimental observations.

Converting self-assembled gold nanoparticle/dendrimer nanodroplets into horseshoe-like nanostructures by thermal annealing

A simple and effective nonlithographic method to produce a novel organization of noble metal nanoparticles into horseshoe-like nanostructures via self-assembly is described. The adsorption of Au nanoparticles stabilized with the dendrimer 1,2,3,4,5,6-hexakis[(3',5'-bis(benzyloxy)benzyl)sulfanylmethyl]benzene (S(6)G(1)) on hydrophilic surfaces (native oxide-terminated Si(111)) resulted in the formation of spatially correlated droplet aggregates. Annealing of Au/S(6)G(1) in thin films caused amalgamated droplets to form arrays of horseshoe-like nanostructures with an average size of approximately 250 nm and an average height of 13 nm. The mobility and the manner in which the semicapped Au nanoparticles are distributed on the hydrophilic substrate are believed to be the promoters that control the growth of the nucleation to create the horseshoe-like structures. Atomic force microscopy (AFM) measurements demonstrated the changes in height and size of the nanoparticles before and after the annealing process. Oxygen plasma etching was used to remove the S(6)G(1) dendrimer to reveal the orientation of the Au nanocrystals in the nanostructure matrix.

Dewetting dynamics in miscible polymer-polymer thin film mixtures

Thin polystyrene films supported by oxidized silicon (SiOx/Si) substrates may be unstable or metastable, depending on the film thickness, h, and can ultimately dewet the substrate when heated above their glass transition. In the metastable regime, holes nucleate throughout the film and subsequently grow due to capillary driving forces. Recent studies have shown that the addition of a second component, such as a copolymer or miscible polymer, can suppress the dewetting process and stabilize the film. We examined the hole growth dynamics and the hole morphology in thin film mixtures composed of polystyrene and tetramethyl bisphenol-A polycarbonate (TMPC) supported by SiOx/Si substrates. The hole growth velocity decreased with increasing TMPC content beyond that expected from changes in the bulk viscosity. The authors show that the suppression of the dewetting velocity is primarily due to reductions in the capillary driving force for dewetting and to increased friction at the substrate-polymer interface. The viscosity, as determined from the hole growth dynamics, decreases with decreasing film thickness, and is connected to a depression of the glass transition of the film.

Nanometer-scale decomposition of ultrathin multilayered polyelectrolyte films

We report that ultrathin multilayered films fabricated from plasmid DNA and synthetic polyamines undergo nanometer-scale transformations that resemble spinodal decomposition when incubated in aqueous media. The patterns and structures generated by this transformation are similar to those observed for the spinodal dewetting of thin films of conventional polymers. This behavior has not, however, been observed for this class of multilayered assemblies, for which long-range electrostatic interactions play significant roles in governing film structure and stability. We demonstrate that it is possible to promote this behavior, prevent it, or control it by varying polymer structure, film composition, or the conditions to which these materials are exposed. These results suggest the basis of methods that could prove useful for the generation of nanostructure on complex surfaces and contribute to methods for the localized delivery of DNA from surfaces.

Dewetting of thin polystyrene films under confinement

The dewetting behavior of thin polystyrene (PS) film has been investigated by placing an upper plate with a ca. 140 nm gap from the underlying substrate with the spin-coated thin polymer films. Three different kinds of dewetting behaviors of thin PS film have been observed after annealing according to the relative position of the PS film to the upper plate. Since the upper plate is smaller than the underlying substrate, a part of the polymer film is not covered by the plate. In this region (I), thin PS film dewetting occurs in a conventional manner, as previously reported. While in the region covered by the upper plate (III), the PS film exhibits unusual dewetted patterns. Meanwhile, in the area right under the edge of the plate (II) (i.e., the area between region I and region III), highly ordered arrays of PS droplets are formed. Formation mechanisms of different dewetted patterns are discussed in detail. This study may offer an effective way to improve the understanding of various dewetting behaviors and facilitate the ongoing exploration of utilizing dewetting as a patterning technique.

Advances on the nanostructuration of magnetic molecules on surfaces: the case of single-molecule magnets (SMM)

SMMs exhibit slow magnetization relaxation rates characteristic of nanodomain particles whose origin is however on individual molecules. For this reason, they have attracted much interest due to their potential applications in high-density information storage devices and quantum computing applications, where for instance, each molecule can be used as a magnetic bit of information. However, for this to become a reality, several basic studies such as their deposition on surfaces are still highly required. Here we will revise all the experimental approximations that have been so far reported for their addressing, nanostructuration and study on surfaces, from the use of stamps as templates to their anchorage to gold surface through the use of thiol-based ligands. It is also important to emphasize that the results and methodologies described along this review are applicable not only to SMMs but to any molecular material.

From a two-dimensional chemical pattern to a three-dimensional topology through selective inversion of a liquid-liquid bilayer

Soft organic surfaces with more and more complex topologies are required daily to engineer appropriate microstructures for many different applications such as DNA array technology, biological optics for advanced photonic systems and microfluidics. Complementarily to conventional lithographic processes, several pioneering methods have been developed recently, by controlling phase separation of polymer blends, spinodal decomposition of homopolymers or by using the action of additional external forces driving diverse instabilities. Here we present a method that not only provides original concepts towards the three-dimensional (3D) structuring of liquids, on the basis of the synergistic effects of molecular diffusion and confined nucleation, but also suggests original solutions for the transport, mixing and filtering of small volumes of liquid. Through the intrinsic destabilization of a liquid-liquid bilayer, the 2D pattern of a chemically structured surface with 'hydrophilic' and 'hydrophobic' domains is transferred to a solid/liquid interface as a 3D topography with either 'positive' or 'negative' replication. This easy-to-use process has potential applications in various technological realms requiring a specific topography at interfaces such as microfluidics or biosensors.

Density variation-induced sign change of the effective hamaker constant in continuum theory

The density variation-induced dewetting model is reformulated in terms of the Dzyaloshinskii-Lifshitz-Pitaevskii continuum theory for van der Waals interactions. Though the density dependence is more complicated than in the London-Hamaker microscopic theory, numerical calculations for model polystyrene films show that the two theories yield qualitatively similar results. The physics of the dewetting model is thus not changed. Quantitatively, the differences between the two theories are notable. The calculations also show that a linear approximation suffices to take density variations into account, thus simplifying the density variation-induced dewetting models.

Robust nanopatterning by laser-induced dewetting of metal nanofilms

We have observed nanopattern formation with robust and controllable spatial ordering by laser-induced dewetting in nanoscopic metal films. Pattern evolution in Co film of thickness 1≤h≤8 nm on SiO(2) was achieved under multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the formation of cellular patterns which evolve into polygons that eventually break up into nanoparticles with unimodal size distribution and short range ordering in nearest neighbour spacing R. Spatial ordering was attributed to a hydrodynamic thin film instability and resulted in a predictable variation of R and particle diameter D with h. The length scales R and D were found to be independent of the laser energy. These results suggest that spatially ordered metal nanoparticles can be robustly assembled by laser-induced dewetting.

Self-organization of triblock copolymer patterns obtained by drying and dewetting

Self-organized block copolymer structures derived from dewetting of thin films are becoming important in nanotechnology because of the various spontaneous and regular sub-micrometric surface patterns that may be obtained. Here, we report on the self-organization of a poly(styrene)-b-poly(ethene-co-butene-1)-b-poly(styrene) triblock copolymer during drying of its solution over a mica substrate. Regular submicrometric arrangements with long-range order were formed at critical polymer concentrations, consisting of parallel ribbons and hexagonal arrays of dots (droplets). This variety of highly ordered structures is explained by the interplay between forming mechanisms, mainly due to "fingering instabilities" at the three-phase line of the copolymer solution during drying. The thickness of the structures was "quantized" due to the microphase separation of the block copolymer. The formation of hexagonal patterns may be attributed to Marangoni instability at the liquid film surface prior to dewetting.

Self-organization of triblock copolymer patterns obtained by drying and dewetting

Self-organized block copolymer structures derived from dewetting of thin films are becoming important in nanotechnology because of the various spontaneous and regular sub-micrometric surface patterns that may be obtained. Here, we report on the self-organization of a poly(styrene)-b-poly(ethene-co-butene-1)-b-poly(styrene) triblock copolymer during drying of its solution over a mica substrate. Regular submicrometric arrangements with long-range order were formed at critical polymer concentrations, consisting of parallel ribbons and hexagonal arrays of dots (droplets). This variety of highly ordered structures is explained by the interplay between forming mechanisms, mainly due to "fingering instabilities" at the three-phase line of the copolymer solution during drying. The thickness of the structures was "quantized" due to the microphase separation of the block copolymer. The formation of hexagonal patterns may be attributed to Marangoni instability at the liquid film surface prior to dewetting.

Deposition of magnetic colloidal particles on graphite and mica surfaces driven by solvent evaporation

The deposition of colloidal magnetite particles onto graphite and mica surfaces induced by solvent evaporation is studied using atomic force microscopy. After evaporation under ambient conditions we observe polydisperse beadlike aggregates; the mean aggregate diameter is larger on graphite than on mica. After evaporation at elevated temperatures we observe a variety of effects, including enhanced particle aggregation and spinodal-like deposition patterns. To explain these trends, we propose mechanisms involving the wetting properties of the solvent. We have also made a brief study of the effects of applied magnetic fields on the formation of aggregates. A field applied parallel to the surface enhances aggregation and favors deposition patterns characteristic of hole-nucleation processes. A perpendicular field leads to a reduction in aggregate size and favors a homogeneous distribution of particles on the surface. These effects are explained in terms of the likely orientation of the dipolar particles on the surface.

Nanostructured collagen layers obtained by adsorption and drying

The supramolecular organization of collagen adsorbed from a 7 microg/ml solution on polystyrene was investigated as a function of the adsorption duration (from 1 min to 24 h) and of the drying conditions (fast drying under a nitrogen flow, slow drying in a water-saturated atmosphere). The morphology of the created surfaces was examined by atomic force microscopy (AFM), while complementary information regarding the adsorbed amount and the organization of the adsorbed layers was obtained using radioassays, X-ray photoelectron spectroscopy (XPS), and wetting measurements. The collagen adsorbed amount increased up to an adsorption duration of 5 h and then leveled off at a value of 0.9 microg/cm2. For samples obtained by fast drying, modeling of the N/C ratios obtained by XPS in terms of thickness and surface coverage, in combination with the adsorbed amount, water contact angle measurements and AFM images, indicated that the adsorbed layer formed a felt starting from 30 min of adsorption, the density and/or the thickness of which increased with the adsorption time. Upon slow drying, the collagen layers formed after adsorption times up to about 2 h underwent a strong reorganization. The obtained nanopatterns were attributed to dewetting, the liquid film being ruptured and adsorbed collagen being displaced by the water meniscus. At higher adsorption times, the organization of the collagen layer was similar to that obtained after fast drying, because the onset of dewetting and/or collagen displacement were prevented by the high density of the collagen felt.

Spinodal instability and pattern formation in thin liquid films confined between two plates

The instability, morphology and pattern formation engendered by the van der Waals force in a thin liquid film of thickness h confined between two closely placed solid surfaces (at distance d > h) are investigated based on nonlinear 3D simulations. The initial and the final stages of dewetting and pattern formation are found to be crucially dependent on the volumetric (thickness) ratio of air and liquid and its deviation from the location of the maximum of the spinodal parameter versus volumetric ratio curve. On a low energy surface, relatively thinner films and wider air gaps favor initial dewetting of the lower plate by the formation of holes, whereas thicker films with thinner air gaps initially evolve by the formation of columns/bridges that join the upper plate. In the later stage of evolution, the initial holes in thinner films evolve into columns/drops, while a rapid coalescence of columns in the thicker films eventually causes formation of holes. Thus, a phase inversion, either from liquid-in-air to air-in-liquid dispersion or vice versa, occurs during the final stages of evolution. A thin film confined between two high-energy solid surfaces forms columns (bridges) only when its mean thickness, h0, is greater than a critical thickness (hc) or the air gap is smaller than a critical distance. The patterns can be aligned by using a topographically patterned confining surface. Conditions on pattern periodicity, amplitude, and the volumetric ratio of air and liquid in the gap are explored for the formation of various types of ordered patterns including annular rings of columns, concentric ripples, parallel channels and a rectangular array of complex features. The results are of significance in soft lithographies such as LISA, soft stamping and capillary force lithography.

Unconventional spinodal surface fluctuations on polymer films

We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability can be adjusted with the film thickness, h0. For the most unstable film studied (whose /(h0 - h(sp))/h(sp)/ = 0.988; h(sp) is the thickness where the second derivative of the interfacial potential of the film equals zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. When the film thickness is increased such that /(h0 - h(sp))/h(sp)/ < or = 0.977, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding the provision of the known corrections from nonlinear effects and thermal noise. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which are not considered in the conventional treatments.

Ultrathin film deposition by liquid CO2 free meniscus coating-uniformity and morphology

Ultrathin organic films of sucrose octaacetate (SOA) were deposited on 12.5 cm diameter silicon wafer substrates using high-pressure free meniscus coating (hFMC) with liquid CO2 (l-CO2) as a coating solvent. The dry film thickness across the wafer and the morphology of deposited films were characterized as a function of coating conditions-withdrawal velocity, solution concentration, and evaporation driving force (deltaP). When no evaporation driving force was applied (deltaP = 0), highly uniform films were deposited with thickness in the range of 8-105 angstroms over the entire concentration range (3-11 wt%). Uniform films were also obtained at low concentrations (3-5 wt%) with a low evaporation driving force (deltaP = 0.0138 MPa). However, films deposited at medium to high concentrations (7-11 wt%) were thicker (110-570 angstroms) and less uniform, with larger nonuniformities at higher applied evaporation driving forces. Optical microscopy and atomic force microscopy (AFM) were used to characterize film morphology including drying defects and film roughness. Films deposited without evaporation had no apparent drying defects and very low root-mean-square (RMS) roughness (1.4-3.8 angstroms). Spinodal-like dewetting morphologies including holes with diameters in the range of 100-300 nm, and surface undulations were observed in films deposited at medium concentration (7 wt%) and low deltaP (0.0138-0.0276 MPa). At higher concentrations and higher evaporative driving forces, spinodal-like dewetting morphologies disappeared but concentric ring defect structures were observed with diameters in the range 20-125 microm. The film thickness and morphology of SOA films deposited from 1-CO2 hFMC were compared to those deposited from toluene and acetone under normal dip coating. Films deposited from l-CO2 hFMC were much thinner, more uniform, and exhibited much fewer drying defects and lower RMS roughness.

Self-organization of Rotaxane Thin Films into Spatially Correlated Nanostructures: Morphological and Structural Aspects

The self-organization of rotaxane thin films into spatially correlated nanostructures is shown to occur upon a thermal stimulus. The mechanism of formation of nanostructures and their organization has been investigated using atomic force microscopy, bright field transmission electron microscopy, selected area electron diffraction, and molecular mechanics simulations. The evolution of the nanostructures follows a complex pathway, where a rotaxane thin film first dewets from the substrate to form nanosized droplets. Droplets coalesce by ripening, generating spatially correlated motifs. In a later stage, the larger droplets change shape, nucleate, and coalesce to yield crystallites that grow into larger crystals by incorporating the surrounding droplets. The results show the following: (i) the nanostructures represent a metastable state of a crystallization process; (ii) spatial correlations emerge during ripening, but they are destroyed as stable nuclei are formed and crystallization proceeds to completion; iii) crystallization, either on graphite or amorphous carbon films, leads to a precise basal plane, viz. (010), which has minimum surface energy. The inherent degrees of freedom permitted in the rotaxane architecture favors the re-organization and nucleation of the film in the solid state. Low-energy trajectories leading to crystallites with stable surfaces and minimum energy contact plane are found to occur via concerted, small amplitude, internal motions without disruption of packing and intermolecular contacts.

Domain shape relaxation and local viscosity in stratifying foam films

We studied the dynamics of two different types of domain shape relaxation in a stratifying foam film composed of an anionic polymer and cationic surfactant. Those films thin in stepwise fashion: circular domains of lower film thickness are formed, expand and coalesce until they cover the whole film surface. We found that the shape relaxation of coalescing domains is governed only by 2D dissipation, and the measurement of the time scales allows to determine the ratio between the driving force (line tension) and local film viscosity. Further, we analyzed the withdrawal of stripes and modeled it by a moving disc pulled by an external force. Here, 3D dissipation can not be neglected (Stokes paradox) and the equilibrium velocity depends logarithmically on the viscosity of the surrounding 3D air. The evaluation of both kinds of relaxation events yields the orders of magnitude of film viscosity and line tension. For the investigated system we found that the film viscosity is at least 30 times larger than the bulk viscosity, which can be explained by the local molecular ordering and strong interactions with film surfaces.

Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements

The hairy attachment system on a gecko's toes, consisting of one billion spatulae in the case of Gekko gecko [Ruibal, R. & Ernst, V. (1965) J. Morphol. 117, 271-294], allows it to adhere to nearly all surface topographies. The mechanistic basis for gecko adhesion has been intensely investigated, but the lowest hierarchical level, that of the spatula, has become experimentally accessible only recently. This report details measurements of the adhesion force exerted by a single gecko spatula for various atmospheric conditions and surface chemistries. Through judicious choice and modification of substrates, the short- and long-range adhesive forces are separated. In contrast to previous work [Autumn, K., Sitti, M., Liang, Y. C. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, J. N. & Full, R. J. (2002) Proc. Natl. Acad. Sci. USA 99, 12252-12256], our measurements clearly show that humidity contributes significantly to gecko adhesion on a nanoscopic level. These findings are crucial for the development of artificial biomimetic attachment systems.

New slip regimes and the shape of dewetting thin liquid films

We compare the flow behavior of liquid polymer films on silicon wafers coated with either octadecyl-(OTS) or dodecyltrichlorosilane (DTS). Our experiments show that dewetting on DTS is significantly faster than on OTS. We argue that this is tied to the difference in the solid/liquid friction. As the film dewets, the profile of the rim advancing into the undisturbed film is monotonically decaying on DTS but has an oscillatory structure on OTS. For the first time we can describe this transition in terms of a lubrication model with a Navier-slip condition for the flow of a viscous Newtonian liquid.

Pinning of a contact line on nanometric steps during the dewetting of a terraced substrate

We study the dewetting of polystyrene films on an alumina surface. We show that the morphology of dewetting holes is drastically modified by the nanometric steps on the surface. Nevertheless, below a critical step height of the order of the polymer chain dimension, the contact line is not anymore sensitive to the defects. This method thus gives an estimation of the limit of validity of macroscopic descriptions of wetting when going down to molecular dimensions.

Partial dewetting of polyethylene thin films on rough silicon dioxide surfaces

The effect of roughness on the dewetting behavior of polyethylene thin films on silicon dioxide substrates is presented. Smooth and rough silicon dioxide substrates of 0.3 and 3.2-3.9 nm root-mean-square roughness were prepared by thermal oxidation of silicon wafers and plasma-enhanced chemical vapor deposition on silicon wafers, respectively. Polymer thin films of approximately 80 nm thickness were deposited by spin-coating on these substrates. Subsequent dewetting and crystallization of the polyethylene were observed by hot-stage optical microscopy in reflection mode. During heating, the polymer films melt and dewet on both substrates. Further observations after cooling indicate that, whereas complete dewetting occurs on the smooth substrate surface, partial dewetting occurs for the polymer film on the rough surface. The average thickness of the residual film on the rough surface was determined by ellipsometry to be a few nanometers, and the spatial distribution of the polymer in the cavities of the rough surface could be obtained by X-ray reflectometry. The residual film originates from the impregnation of the porous surface by the polymer fluid, leading to the observed partial dewetting behavior. This new type of partial dewetting should have important practical consequences, as most real surfaces exhibit significant roughness.