Spontaneous Dewetting Research Articles

Consequences of Interfacial Viscoelasticity on Thin Film Stability

The phenomenon of dewetting is frequently observed in our everyday life. It is of central importance in many technological applications as well as in a variety of physical and biological systems. The presence of nonsoluble surfactants at an air/liquid interface may affect the dewetting properties of the aqueous layer. An important example is the tear film, which comprises an aqueous layer covered with a ∼100-nm-thick blanket of lipids, known as the meibomian lipids. Interfacial rheological measurements of meibomian lipids reveal that these films are remarkably viscoelastic. Tear film dewetting is of central importance to understanding tear film stability. To better understand the role of surface viscoelasticity in tear film stability, we have developed a methodology to systematically control interfacial rheology of thin aqueous layers at the onset of dewetting events. The apparatus allows control over the surface pressure of the monolayer, which is a key feature since this variable controls the surface viscoelasticity. Three insoluble monolayer materials were used: newtonian arachidyl alcohol (AA), DPPC, a phospholipid that is slightly viscoelastic, and meibum, which produces a strongly viscoelastic monolayer. It is reported that monolayers of viscoelastic surfactants are able to stabilize thin films against spontaneous dewetting. As the surface pressure of these layers is increased, their effectiveness is enhanced. Moreover, these surfactants are able to reduce the critical film thickness for dewetting. Meibum is particularly effective in stabilizing thin films. Our results suggest that the meibomian lipids play a vital role in maintaining tear film stability in addition to suppressing evaporation.

Micropatterning of Polymer Brushes: Grafting from Dewetting Polymer Films for Biological Applications

In this novel platform, a micropatterned polymer brush was obtained by grafting poly(poly(ethylene glycol) methyl ether methacrylate) (poly(PEGMA)) from a thin macroinitiator film using atom transfer radical polymerization (ATRP). A pattern of holes was formed in the macroinitiator film by taking advantage of its spontaneous dewetting above the glass transition temperature from a bottom polystyrene film, driven by unfavorable intermolecular forces. Patterning by dewetting can be achieved at length-scales from a few hundred nanometers to several tens of micrometers, by simply thermally annealing the bilayer above the glass transition temperature of the polymer. This approach is substrate-independent, as polymer films can be cast onto surfaces of different size, shape, or material. As a demonstration of its potential, proteins, and individual cells were attached on targeted bioadhesive polystyrene areas of the micropatterns within poly(PEGMA) protein-repellent brushes. We anticipate this approach will be suitable for the patterning of brushes, especially for biomedical applications such as in the study of single cells and of cell cocultures.

Directed deposition of silicon nanowires using neopentasilane as precursor and gold as catalyst.

In this work the applicability of neopentasilane (Si(SiH3)4) as a precursor for the formation of silicon nanowires by using gold nanoparticles as a catalyst has been explored. The growth proceeds via the formation of liquid gold/silicon alloy droplets, which excrete the silicon nanowires upon continued decomposition of the precursor. This mechanism determines the diameter of the Si nanowires. Different sources for the gold nanoparticles have been tested: the spontaneous dewetting of gold films, thermally annealed gold films, deposition of preformed gold nanoparticles, and the use of “liquid bright gold”, a material historically used for the gilding of porcelain and glass. The latter does not only form gold nanoparticles when deposited as a thin film and thermally annealed, but can also be patterned by using UV irradiation, providing access to laterally structured layers of silicon nanowires.

A hydrophobic entrance enhances ion current rectification and induces dewetting in asymmetric nanopores

Hydrophobic interactions and local dewetting of hydrophobic cavities have been identified as a key mechanism for ionic gating in biological voltage-gated channels in a cell membrane. Hydrophobic interactions are responsible for rectification of the channels, i.e. the ability to transport ions more efficiently in one direction compared to the other. We designed single polymer nanopores with a hydrophobic gate on one side in the form of a single layer of C10 or C18 thiols. This nanoporous system behaves like an ionic diode whose direction of rectification is regulated by the pH of the electrolyte. In addition, reversible dewetting of the hydrophobic region of the pore was observed as voltage-dependent ion current fluctuations in time between conducting and non-conducting states. The observations are in accordance with earlier molecular dynamics simulations, which predicted the possibility of spontaneous and reversible dewetting of hydrophobic pores.

Growth of Noncircular and Faceted Holes in Liquid–Liquid Dewetting of Thin Polymer Bilayers

Although the formation of circular holes is the norm in the spontaneous dewetting of thin polymer films, we show highly anisotropic hole growth leading to irregular and faceted shapes in dewetting of the upper polystyrene layer in a thin polymer bilayer (silica–PMMA–PS–air). The irregular shapes are formed when viscous dissipation is dominated by the upper PS dewetting layer under the conditions of its much higher viscosity and lower thickness or the lower viscosity and higher thickness of the substrate PMMA layer. These conditions engender strong slippage of the dewetting layer, resulting in very broad rims and the long-range viscous interactions among randomly distributed, initially circular holes. The growth occurs selectively in the directions of less viscous resistance. Thus, the irregular growth of holes can be inhibited by increasing the PS film thickness or by decreasing the PMMA film thickness.

Large-area Raman enhancement substrates using spontaneous dewetting of gold films and silver nanoparticles deposition

We present a cost-effective method for making large-area surface enhanced Raman scattering (SERS) substrates by using spontaneous dewetting of ultrathin gold film. The dewetting of 5-nm-thick gold film formed high-density gold islands ranged from 40 nm to 80 nm. The measured SERS signal was 5 times stronger than synthesised gold nanoparticles. The SERS signals can be further increased by depositing small silver nanoparticles on the dewetted film. The experimental results showed 5-nm-thick silver coating increased SERS signals up to 10 times. The calculations by finite-difference time-domain method verified such SERS enhancement originated from enhanced electric fields between gold islands and silver nanoparticles. In the application, we demonstrated an all-optical measurement of pH values in microfluidic devices by using SERS signals of para-mercaptobenzoic acid.

Thermodynamic model for the dewetting instability in ultrathin films

The spontaneous pattern formation via the classical spinodal dewetting instability in ultrathin films is a nonlinear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the film’s dynamics. Alternately, the thermodynamic (TH) approach based on equating the rate of free energy decrease to the rate of frictional loss via viscous dissipation [de Gennes, C. R. Acad. Paris 298, 111 (1984)] can give similar information. Here we have evaluated dewetting in the presence of film-thickness- (h) dependent thermocapillary forces. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h-dependent temperature. The TH approach provides an analytical description of this thermocapillary dewetting. The results of this approach agree with those from linear theory and experimental observations provided the minimum dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss.

Influence of Nanoroughness on Contact Line Motion

The dependence of wetting behavior on roughness is of great relevance in many practical applications. However, most studies concentrate on static wettability. In this study, the influence of nanoroughness on the spontaneous dewetting of water is investigated. For both smooth and nanorough surfaces, contact line motion is governed by two distinct regimes: hydrodynamic dissipation dominates for small water contact angles, while the molecular kinetic theory describes contact line motion at large contact angles. In a quantitative comparison of nanorough with chemically heterogeneous surfaces, the similarities in behavior suggest that there may be a universal dewetting mechanism.

Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films

We show here that the morphological pathway of spontaneous dewetting of ultrathin Ag films onSiO2 under nanosecond laser melting is dependent on film thickness. For films with thicknessh of2 nm ≤ h ≤ 9.5 nm, themorphology during the intermediate stages of dewetting consisted of bicontinuous structures. For filmswith 11.5 nm ≤ h ≤ 20 nm, the intermediate stages consisted of regularly sized holes. Measurement of thecharacteristic length scales for different stages of dewetting as a function of filmthickness showed a systematic increase, which is consistent with the spinodaldewetting instability over the entire thickness range investigated. This change inmorphology with thickness is consistent with observations made previously forpolymer films (Sharma and Khanna 1998 Phys. Rev. Lett. 81 3463–6; Seemann et al 2001 J. Phys.: Condens. Matter 13 4925–38). Based on the behavior of freeenergy curvature that incorporates intermolecular forces, we have estimated themorphological transition thickness for the intermolecular forces for Ag onSiO2. The theory predictions agree well with observations for Ag. These results show that it ispossible to form a variety of complex Ag nanomorphologies in a consistent manner, whichcould be useful in optical applications of Ag surfaces, such as in surface enhanced Ramansensing.

Spontaneous dewetting-induced residue-free patterning at room temperature

A lithographic patterning method is presented that is based on dewetting induced by sequential molding under an applied pressure. Because of spontaneous dewetting taking place, the window to be opened is free from any residue and the surface exposure is instantaneously assured. This residue-free patterning can be accomplished without any heating process and surface treatment, irrespective of pattern duty ratio. The residue-free patterning is made possible with the use of a rigiflex mold and a roller that is used to bring about pressure-induced thinning leading to spontaneous dewetting. A necessary condition for the method is that the spreading coefficient of spin-coated liquid be negative. The exposed surface can be utilized as a sacrificial layer for etching of underlying layer and/or thin film deposition in a fabrication of electronic and biological devices.

Energy driven self-organization in nanoscale metallic liquid films

Nanometre thick metallic liquid films on inert substrates can spontaneously dewet and self-organize into complex nanomorphologies and nanostructures with well-defined length scales. Nanosecond pulses of an ultraviolet laser can capture the dewetting evolution and ensuing nanomorphologies, as well as introduce dramatic changes to dewetting length scales due to the nanoscopic nature of film heating. Here, we show theoretically that the self-organization principle, based on equating the rate of transfer of thermodynamic free energy to rate of loss in liquid flow, accurately describes the spontaneous dewetting. Experimental measurements of laser dewetting of Ag and Co liquid films on SiO(2) substrates confirm this principle. This energy transfer approach could be useful for analyzing the behavior of nanomaterials and chemical processes in which spontaneous changes are important.

Growth of Ca3Co4O9 films: Simple chemical solution deposition and stress induced spontaneous dewetting

Chemical solution deposition as a simple method for preparation of oxide films is used to fabricate misfit structured Ca3Co4O9 thermoelectric films using cheap precursors, Ca and Co acetates, which opens up a simple route to fabricate large-area Co-based oxide thermoelectric films. The results show that when the annealing temperature is higher than 800°C crystallized Ca3Co4O9 films can be obtained on single-crystal LaAlO3 substrates, whereas the Ca3Co4O9 films are amorphous for lower annealing temperatures. Experimental evidence shows that the amorphous Ca3Co4O9 films behave like semiconductors with enhanced resistivity and Seebeck coefficient within the measured temperature range. The magnetoresistance is positive indicating the absence of long-range order of the Co-ion spins. For the crystallized Ca3Co4O9 films, it can be seen that the thinner films show the characteristics of spontaneous dewetting induced by the stress, which suggests that it is possible to prepare nanostructured Ca3Co4O9 films through tailoring the film thickness and the lattice mismatch; whereas, for the thicker films the temperature dependence of the resistivity and the Seebeck coefficient shows that it is possible to prepare excellent Ca3Co4O9 thermoelectric films using chemical solution deposition route, which will broaden the range of preparation methods for misfit structured Co-based thermoelectric films.

Dynamics of Dewetting at the Nanoscale Using Molecular Dynamics

Large-scale molecular dynamics simulations are used to model the dewetting of solid surfaces by partially wetting thin liquid films. Two levels of solid-liquid interaction are considered that give rise to large equilibrium contact angles. The initial length and thickness of the films are varied over a wide range at the nanoscale. Spontaneous dewetting is initiated by removing a band of molecules either from each end of the film or from its center. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Consistent with the current understanding of wetting dynamics, film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. In addition, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These new results imply not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the effective viscosity of the film decreases with thickness.

Stress-induced spontaneous dewetting of heteroepitaxialYBa2Cu3O7thin films

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Complex aggregates of silica microspheres by the use of a polymer template

Evaporation of a droplet of silica microsphere suspension on a polystyrene and poly(methyl methacrylate) blend film with isolated holes in its surface has been exploited as a means of particles self-assembly. During the retraction of the contact line of the droplet, spontaneous dewetting combined with the strong capillary force pack the silica microspheres into the holes in the polymer surface. Complex aggregates of colloids are formed after being exposed to acetone vapor. The morphology evolution of the underlying polymer film by exposure to acetone solvent vapor is responsible for the complex aggregates of colloids formation.

Particle nucleation for dewetting initiation of thin polymer films

AbstractA quantitative comparison between spontaneous dewetting and particle nucleation for thin (thickness = 17nm) polystyrene (PS) films on nonwettable silicon (Si) surfaces is presented both experimentally and theoretically. Performing experiments in a class 100 clean room, we found that ∼ 23% of the observed dry patches formed because of dust particles, while the majority of the holes formed via the well known spontaneous dewetting process. The result was verified qualitatively by diffusion theory, which, however, predicted a diminished role for the airborne particles, leading to the conclusion that pre‐existing particles on the Si surfaces and/or the polymer solutions contribute substantially to the dewetting process. The driving force of particle motion into the polymer film is examined by placing aluminum oxide (Al2O3) particles on PS films. Finally, the effect of particle geometry is studied by placing gold (Au) disks on the free surface of PS films. An optically continuous PS film is found to be present around the periphery of the disk particles, even after the completion of the dewetting process in the rest of the sample. An attempt to explain dewetting inhibition at the vicinity of the micro‐disks, on the basis of molecular interactions developed in the system Au/PS/Si, is finally presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 138–145, 2005

Assembly of Colloidal Particles by Evaporation on Surfaces with Patterned Hydrophobicity

Drops containing suspended particles are placed on surfaces of patterned wettability created using soft lithography; the drop diameter is large compared to the dimensions of the patterns on the substrate. As the three-phase contact line of the drop recedes, spontaneous dewetting of the hydrophobic domains and flow into the hydrophilic domains create discrete fluid elements with peripheries that can mimic the underlying surface topography. Suspended particles are carried with the fluid into the wetted regions and deposit there as the discrete fluid domains evaporate. If particle volume fractions are sufficiently high, the entire wetted domain can be covered with colloidal crystals. At lower volume fractions, flow within the evaporating fluid element can direct the deposition of colloidal particles at the peripheries of the domains. High-resolution arrays of particles were obtained with a variety of features depending upon the relative size of the wetting regions to the particles. When the wetting region is larger than the particles, three-dimensional and two-dimensional arrays of ordered particles mimicking the shape of the wetting pattern form, depending on the particle volume fraction. For lower volume fractions, one-dimensional (1-D) arrays along the wet/non-wet boundaries form. When the particle size is similar to the height of fluid on the wetted domain, zero-dimensional distributions of single particles centered in the wet regions can form for wetted squares or 1-D distributions (stripes) form along the axis of striped domains. Finally, when the wetting region is smaller than the particle size, the particles do not deposit within the features but are drawn backward with the receding drop. These results indicate that evaporation on surfaces of patterned wetting provides a highly parallelizable means of tailoring the geometry of particle distributions to create patterned media.

Direct Observation of Wetting and Diffusion in the Hydrophobic Interior of Silica Nanotubes

Fluorescence microscopy techniques are used to investigate diffusion and wetting at the interior surface of template-synthesized silica nanotubes. Imaging of individual nanotubes reveals that complete modification of the nanotube interiors with hydrophobic ligands is attained during nanotube synthesis. Fluorescence recovery after photobleaching experiments show that (1) spontaneous dewetting of the nanotube interior occurs when the immersing solvent is displaced with pure water and (2) anomalously slow diffusion is observed at the nanotube interior surface.

Dewetting Patterns of Thin Films of Charged Polymer Solutions

Dewetting of thin films of a volatile aqueous solution of charged polymer is investigated using atomic force microscopy. Spontaneous dewetting of films below 10 nm shows a wide range of morphology ranging from holes, polygonal networks, and bicontinuous structures to droplets, depending on film thickness. These morphologies are also shown to depend on polymer charge density and the rate of dewetting. At high polymer charge density, unusual features are observed, including very long fingers around hole rims and cylindrical structures that resist fragmentation. These features, attributed to viscous effects, are enhanced when the dewetting rate is increased.

Wetting transitions at soft, sliding interfaces.

We observe (by optical interferometry) the contact of a rubber cap squeezing a nonwetting liquid against a plate moving at velocity U. At low velocities, the contact is dry. It becomes partially wet above a threshold velocity V(c1), with two symmetrical dry patches on the rear part. Above a second velocity V(c2), the contact is totally wet. This regime U>V(c2) corresponds to the hydroplaning of a car (decelerating on a wet road). We interpret the transitions at V(c1), V(c2) in terms of a competition between (a) liquid invasion induced by shear (b) spontaneous dewetting of the liquid (between nonwettable surfaces).