Dewetting of solids by the formation of holes in macroscopic liquid films

What Controls the Hole Formation of Nanodroplets: Hydrodynamic or Thermodynamic Instability?

Using molecular dynamics simulations, we investigate the air hole formation of water nanodroplets impacting hydrophilic to hydrophobic surfaces in the range of static contact angles from 30° to 140° with different initial surface temperatures ranging from 300 to 1000 K. We show that the hole dynamics of nanodroplets are different from those observed in millimeter-sized droplets. The hole formation can be observed on smooth surfaces for nanodroplets; however, it only occurs on nonsmooth surfaces for millimeter-sized droplets. We clarify that the hole formation of nanodroplets is triggered by a nucleated vapor bubble due to thermodynamic instability, whereas it is initiated by air bubble entrapment during impact due to hydrodynamic instability for millimeter-sized droplets. The hole formation of nanodroplets relies heavily on the surface temperature and surface wettability, because the nucleated vapor bubble more easily occurs and grows on the surface with high initial temperatures and hydrophobic surfaces. Based on the thermal stability analysis, a criterion is developed to predict the hole formation of nanodroplets, which verifies the dependence of hole formation on the surface temperature and wettability. Furthermore, we show that the ring-bouncing of nanodroplets is triggered by the nucleated vapor bubble. We clarify the reasons for the reduced contact time of nanodroplets caused by the ring-bouncing.

Dynamic Aspects of Pre-Soft Contact Lens Tear Film and Their Relation to Dry Eye: Basic Science and Clinical Relevance

Soft contact lens (SCL) perturbs the intimate connection between the pre-lens tear film (PLTF) and the ocular surface in various ways, i.e., (i) decrease in tear meniscus radius and aqueous tear thickness, (ii) attenuation of tear film lipid layer spread, (iii) limited wettability of SCL surface, (iv) increased friction with eyelid wiper, etc. This often results in SCL-related dry eye (SCLRDE) manifested as PLTF instability and contact lens discomfort (CLD). In this review, the individual contributions of factors (i-iv) to PLTF breakup patterns (BUP) and CLD are considered via the tear film-oriented diagnosis framework adopted by the Asia Dry Eye Society from a clinical and basic science perspective. It is shown that SCLRDE (due to aqueous deficiency, increased evaporation, or decreased wettability) and BUP of PLTF classify within the same types as the ones observed for the precorneal tear film. The analysis of PLTF dynamics reveals that the inclusion of SCL enhances the manifestation of BUP associated with (i) decreased thickness of PLTF aqueous layer and (ii) limited SCL wettability as shown by the rapid expansion of BUP area. PLTF thinness and instability result in increased blink-related friction and lid wiper epitheliopathy as major contributor to CLD.

Energy of a Drop Required to Break a Liquid Film

An external disturbance can destabilize and break a liquid film on a nonwettable surface. Previous studies focused on evaluating critical film thickness for a spontaneous breakup, but the required energy has been unknown. We experimentally found that the energy of a drop to break a liquid film is an order of magnitude more than that predicted by a free energy balance. Here, we show how to evaluate the energy needed to rupture a liquid film by considering the formation of a crater with a critical size.

Hydrodynamic constraints on the energy efficiency of droplet electricity generators

Electric energy generation from falling droplets has seen a hundred-fold rise in efficiency over the past few years. However, even these newest devices can only extract a small portion of the droplet energy. In this paper, we theoretically investigate the contributions of hydrodynamic and electric losses in limiting the efficiency of droplet electricity generators (DEG). We restrict our analysis to cases where the droplet contacts the electrode at maximum spread, which was observed to maximize the DEG efficiency. Herein, the electro-mechanical energy conversion occurs during the recoil that immediately follows droplet impact. We then identify three limits on existing droplet electric generators: (i) the impingement velocity is limited in order to maintain the droplet integrity; (ii) much of droplet mechanical energy is squandered in overcoming viscous shear force with the substrate; (iii) insufficient electrical charge of the substrate. Of all these effects, we found that up to 83% of the total energy available was lost by viscous dissipation during spreading. Minimizing this loss by using cascaded DEG devices to reduce the droplet kinetic energy may increase future devices efficiency beyond 10%.

Relationships between the material properties of silicone hydrogels: Desiccation, wettability and lubricity

Silicone hydrogels (SiHy), represent composite matrices composed of hydrophobic gas permeable silicone (Si) rich core and a surface enriched with hydrophilic polymer moieties. Their utilization in contact lens design requires number of SiHy properties (hydration, wettability, lubricity) to be optimized for the challenging conditions at the ocular surface. Typical limitations in literature are that (i) these properties are studied in isolation, monitoring only one parameter but not the rest of them, and (ii) measurements are performed with hydrated samples immediately after removal from storage solutions. Here we study the simultaneous evolution of critical material properties (evaporative loss of water, water contact angle, coefficient of friction) of different SiHy subjected to continuous blink-like desiccation/rehydration cycling. SiHy with wetting agents incorporated in their core (narafilcon A, senofilcon A) were particularly susceptible to extended desiccation. Stenfilcon A, a material with only 3% bulk Si content maintained its performance for 4 h of cycling, and delefilcon A (80% surface water content) resisted extended 8 h of desiccation/rehydration runs. Strong correlation exists between the evolution of SiHy wettability and lubricity at ≥4 h of blink-like cycling. Understanding the interplay between SiHy properties bears insights for knowledge based design of novel ophthalmic materials.

How a raindrop gets shattered on biological surfaces

Rainfall on biological superhydrophobic surfaces is ubiquitous in nature. Previous studies in a laboratory setting have focused only on low-speed impacts, which can be quite different from rain conditions in nature. In this study, we reported unexpected and interesting shock-like patterns when a drop impacts biological surfaces at high speeds. These shock-like waves trigger sudden drop fragmentation into smaller satellite droplets and lead to a more than twofold decrease in contact time. Our findings may elucidate biological advantages (hypothermia risk reduction for birds, flight stability for insects, spore dispersal on plants) of superhydrophobic surfaces triggered by microstructures.

Many biological surfaces of animals and plants (e.g., bird feathers, insect wings, plant leaves, etc.) are superhydrophobic with rough surfaces at different length scales. Previous studies have focused on a simple drop-bouncing behavior on biological surfaces with low-speed impacts. However, we observed that an impacting drop at high speeds exhibits more complicated dynamics with unexpected shock-like patterns: Hundreds of shock-like waves are formed on the spreading drop, and the drop is then abruptly fragmented along with multiple nucleating holes. Such drop dynamics result in the rapid retraction of the spreading drop and thereby a more than twofold decrease in contact time. Our results may shed light on potential biological advantages of hypothermia risk reduction for endothermic animals and spore spreading enhancement for fungi via wave-induced drop fragmentation.

Stochastic induction time of attachment due to the formation of transient holes in the intervening water films between air bubbles and solid surfaces

HYPOTHESIS

Bubble attachment to hydrophobic solid surfaces is influenced by the liquid film instability. Inclusion of transiently formed holes within the film rather than the so-called hydrophobic force in the theory is expected to better describe and explain film rupture and triple contact line formation in the bubble-surface attachment process. The significance of surface hydrophobicity and hole formation renders the stochastic nature of the induction time of attachment.

EXPERIMENTS

A combination of high-speed video microscopy and theoretical analysis was applied to investigate the induction time of attachment and critical film thickness of air bubbles rising freely perpendicularly to silica surfaces of different hydrophobicities.

FINDINGS

Film rupture occurred statistically for shorter induction times and thicker films on the more hydrophobic surface, rejecting the conjecture of hydrophobic force. Computed results of the critical base radius of the transient holes causing film rupture were merged together nicely, independently of surface hydrophobicity. The paper sheds light on the significance of hydrophobicity on the attachment process by means of a novel and easily implemented methodology, without relying on the debatable hydrophobic force.

Influence of dewetting on the crystallization behavior of CuCl₂ in the presence of BSA during evaporation in a Petri dish

The overall structure of dihydrate cupric chloride (CuCl₂ * 2H₂O) crystallization patterns in the presence of bovine serum albumin (BSA) in a Petri dish is influenced by dewetting. The dewetting behavior, which can be either before or after initial CuCl₂ nucleation, depends on the amount of CuCl₂ and BSA in the Petri dish. We postulate that the concentration and/or temperature gradient area in the dish, which is built up during the evaporation process, coincides with the location where dewetting predominantly starts. This hypothesis could be supported by measurements of the CuCl₂ coverage of the Petri dish. During the evaporation the height of the meniscus at the rim of the Petri dish recedes in favor of the central Petri dish area. This could not be explained by the above mentioned hypothesis.

Measurement of Instability of Thin Liquid Films by Synchronized Tri-wavelength Reflection Interferometry Microscope

Film thickness measurement of unstable thin liquid films (TLFs) remains a challenge due to the difficulty in determining the order of fringes prior to the film rupture. In the present work, a synchronized tri-wavelength reflection interferometry microscope (STRIM) was developed and employed to determine the spatiotemporal thickness profiles of the TLFs between air bubbles and various hydrophobic surfaces in 10^-2 M NaCl solutions. Both accuracy and precision of film thickness measurements were found to be better than 3 nm over the range of 0-1 μm. It was found that when the radii of air bubbles were in the range 0.71-0.88 mm, the critical rupture thicknesses of the wetting films formed on hydrophobic quartz surfaces having water contact angles of 95° scattered over a range of 57-335 nm with a medium rupture thickness of 122 nm. For smaller air bubbles with radii of 0.13-0.26 mm, the critical rupture thicknesses were much more narrowly distributed with a medium rupture thickness of 27 nm. The result obtained with the TLFs between two air bubbles, i.e., foam film, showed that the critical rupture thickness was increased from 25 to 40 nm, when the sizes of air bubbles were increased from 220 to 960 μm. Compared to rupture thickness of the foam film, the critical rupture thickness of the TLF between an air bubble and a dodecane droplet was smaller, indicating that the film rupture might be related to the hydrophobicity of interacting surfaces. In addition to attractive surface forces, both wave motions and gas molecules in TLF might be associated with the film rupture.

Dewetting of Thin Liquid Films Surrounding Air Bubbles in Microchannels

As an air bubble translates in a microchannel, a thin film of liquid is formed on the bounding walls. In a microchannel with a rectangular cross-section, the liquid in the film leaks toward the low-pressure corners of the geometry, which leads to the appearance of local minima in the film thickness in the cross-sectional plane. In such a configuration, theory suggests that the minimum film thickness scales with Ca and Ca^4/3 depending on the distance from the nose of the bubble, where Ca = μU_b/γ is the flow capillary number based on the bubble velocity U_b, liquid viscosity μ, and surface tension γ, and Ca ≪ 1. We show that the film of a partially wetting liquid dewets on the channel wall at the sites of the local minima in the film thickness as it acquires thicknesses around and below 100 nm. Our experiments show that the distance L_w between the nose of the bubble and the initial dewetting location is a function of Ca and surface wettability. For channels of different wettability, L_w always scales proportional to Ca^α, where 1.7 < α < 2 for the range of 10^-5 < Ca < 10^-2. Moreover, L_w increases up to 10 times by enhancing the wettability of the surface at a given Ca. Our present measurements of L_w provide a design constraint on the lengths of bubbles to maintain a liquid wet channel without dry patches on the wall.

Rapid Bouncing of High-Speed Drops on Hydrophobic Surfaces with Microcavities

Artificial hydrophobic surfaces that can induce rapid drop detachment have many significant engineering applications from self-cleaning to anti-icing. In the present study, we found that hydrophobic surfaces with microcavities can rapidly induce a center-assisted recoil of high-speed impacting drops and subsequently result in an approximately 40% reduction in contact time compared with conventional superhydrophobic surfaces. More intriguingly, the contact time on these surfaces has a rapid descent of over 50% at high-speed impacts compared with that at low-speed impacts, which is due to the rapid bouncing induced by the faster retraction of the liquid lamella triggered by the instability of air bubbles beneath the center of the colliding drops. We believe that these findings will provide a valuable strategy for designing self-cleaning and anti-icing surfaces by minimizing the contact time of high-speed drops.

Not spreading in reverse: The dewetting of a liquid film into a single drop

Wetting and dewetting are both fundamental modes of motion of liquids on solid surfaces. They are critically important for processes in biology, chemistry, and engineering, such as drying, coating, and lubrication. However, recent progress in wetting, which has led to new fields such as superhydrophobicity and liquid marbles, has not been matched by dewetting. A significant problem has been the inability to study the model system of a uniform film dewetting from a nonwetting surface to a single macroscopic droplet-a barrier that does not exist for the reverse wetting process of a droplet spreading into a film. We report the dewetting of a dielectrophoresis-induced film into a single equilibrium droplet. The emergent picture of the full dewetting dynamics is of an initial regime, where a liquid rim recedes at constant speed and constant dynamic contact angle, followed by a relatively short exponential relaxation of a spherical cap shape. This sharply contrasts with the reverse wetting process, where a spreading droplet follows a smooth sequence of spherical cap shapes. Complementary numerical simulations and a hydrodynamic model reveal a local dewetting mechanism driven by the equilibrium contact angle, where contact line slip dominates the dewetting dynamics. Our conclusions can be used to understand a wide variety of processes involving liquid dewetting, such as drop rebound, condensation, and evaporation. In overcoming the barrier to studying single film-to-droplet dewetting, our results provide new approaches to fluid manipulation and uses of dewetting, such as inducing films of prescribed initial shapes and slip-controlled liquid retraction.

Experimental and numerical insights into isotropic spreading and deterministic dewetting of dielectrowetted films

Dielectrowetting effects of surface wrinkling, isotropic vs anisotropic spreading, electrode geometry, and deterministic dewetting are presented both experimentally and by 3D numerical modeling. The numerical results are generated by COMSOL in conjunction with the phase-field and electrohydrodynamic methods, including comparisons to experimental data. The dynamic behavior of the two-phase system has been accurately characterized on both the macro- and microscopic level. This work provides a deeper theoretical insight into the operating physics of dielectrowetting superspreading devices.

Scaling dielectrowetting optical shutters to higher resolution: microfluidic and optical implications

A detailed study is reported on the implications of scaling dielectrowetting optical shutters to higher resolutions. Reducing droplet sizes from millimeters to 100 μm in diameter increases the relevance of microfluidic physics such as pinning, film breakup, and dewetting speed as well as optical physics such as transmission and diffraction. In addition, in this work we present improved material systems, including optimized dielectric stacks which reduce electrochemical degradation, and blended lower-viscosity fluids which increase dewetting speed. A higher-resolution device of ~250 μm diameter demonstrates switching speeds of <100 ms and a clear, optically transmissive aperture of >70%. In addition to revealing science not previously discussed, this work has strong applied importance as scaling to higher resolutions is desirable for improving visual appearance in applications ranging from smart windows to electronic signage.

Water-evaporation reduction by duplex films: application to the human tear film

Water-evaporation reduction by duplex-oil films is especially important to understand the physiology of the human tear film. Secreted lipids, called meibum, form a duplex film that coats the aqueous tear film and purportedly reduces tear evaporation. Lipid-layer deficiency is correlated with the occurrence of dry-eye disease; however, in-vitro experiments fail to show water-evaporation reduction by tear-lipid duplex films. We review the available literature on water-evaporation reduction by duplex-oil films and outline the theoretical underpinnings of spreading and evaporation kinetics that govern behavior of these systems. A dissolution-diffusion model unifies the data reported in the literature and identifies dewetting of duplex films into lenses as a key challenge to obtaining significant evaporation reduction. We develop an improved apparatus for measuring evaporation reduction by duplex-oil films including simultaneous assessment of film coverage, stability, and temperature, all under controlled external mass transfer. New data reported in this study fit into the larger body of work conducted on water-evaporation reduction by duplex-oil films. Duplex-oil films of oxidized mineral oil/mucin (MOx/BSM), human meibum (HM), and bovine meibum (BM) reduce water evaporation by a dissolution-diffusion mechanism, as confirmed by agreement between measurement and theory. The water permeability of oxidized-mineral-oil duplex films agrees with those reported in the literature, after correction for the presence of mucin. We find that duplex-oil films of bovine and human meibum at physiologic temperature reduce water evaporation only 6-8% for a 100-nm film thickness pertinent to the human tear film. Comparison to in-vivo human tear-evaporation measurements is inconclusive because evaporation from a clean-water surface is not measured and because the mass-transfer resistance is not characterized.

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.

Slip effects in polymer thin films

Probing the fluid dynamics of thin films is an excellent tool for studying the solid/liquid boundary condition. There is no need for external stimulation or pumping of the liquid, due to the fact that the dewetting process, an internal mechanism, acts as a driving force for liquid flow. Viscous dissipation, within the liquid, and slippage balance interfacial forces. Thus, friction at the solid/liquid interface plays a key role towards the flow dynamics of the liquid. Probing the temporal and spatial evolution of growing holes or retracting straight fronts gives, in combination with theoretical models, information on the liquid flow field and, especially, the boundary condition at the interface. We review the basic models and experimental results obtained during the last several years with exclusive regard to polymers as ideal model liquids for fluid flow. Moreover, concepts that aim to explain slippage on the molecular scale are summarized and discussed.

Rupture of thin films formed during droplet impact

Rupture of liquid films formed during droplet impact on a dry solid surface was studied experimentally. Water droplets (580±70 μm) were photographed as they hit a solid substrate at high velocities (10–30 m s −1 ). Droplet–substrate wettability was varied over a wide range, from hydrophilic to superhydrophobic, by changing the material of the substrate (glass, Plexiglas, wax and alkylketene dimer). Both smooth and rough wax surfaces were tested. Photographs of impact showed that as the impact velocity increased and the film thickness decreased, films became unstable and ruptured internally through the formation of holes. However, the impact velocity at which rupture occurred was found to first decrease and then increase with the liquid–solid contact angle, with wax showing rupture at all impact velocities tested. A thermodynamic stability analysis combined with a droplet spreading model predicted the rupture behaviour by showing that films would be stable at very small or at very large contact angles, but unstable in between. Film rupture was found to be greatly promoted by surface roughness.

The insect-trapping rim of Nepenthes pitchers: surface structure and function

Carnivorous pitcher plants of the genus Nepenthes capture prey with a pitfall trap that relies on a micro-structured, slippery surface. The upper pitcher rim (peristome) is fully wettable and causes insects to slip by aquaplaning on a thin water film. The high wettability of the peristome is probably achieved by a combination of hydrophilic surface chemistry, surface roughness and the presence of hygroscopic nectar. Insect foot attachment could be prevented by the delayed drainage of the thin water film between the adhesive pad and the surface. Drainage should be faster for insects with a hairy adhesive system; however, they slip equally on the wet peristome. Therefore the stability of the water film against dewetting appears to be the key factor for aquaplaning. New experimental techniques may help to clarify the detailed function of the pitcher plant peristome and to explore its potential for biomimetic applications.

Dewetting Revisited: New Asymptotics of the Film Stability Diagram and the Metastable Regime of Nucleation and Growth of Dry Zones

Stability properties of a nonwetting film are discussed. Assuming a general form of the disjoining pressure, accurate asymptotic formulas for the upper thickness range of the film instability/metastability are derived. This analysis is applied to two particular cases: a nonionic liquid film with the (m, n) power form of the disjoining pressure and an ionic liquid film with an exponentially decaying electrostatic part of the disjoining pressure. The metastable regime of dewetting is considered, and an expression for the critical radius of a hole is derived. A new Fokker-Planck kinetic model of metastable dewetting, applicable at early stages of the process, is developed. It yields a relationship between the number of viable holes (per unit area and unit time) moving in steady-state regime to the supercritical part of the "embryo size space" and the equilibrium number of "critical" holes determined from thermodynamics. The dynamics of metastable dewetting is quantitatively described in terms of the surface fraction of holes in the film. Continuous dynamic models of the metastable dewetting applicable in the entire range of times have to include the thermal noise, as proposed by V. S. Mitlin (1994, Colloids Surf. A 89, 97). Copyright 2000 Academic Press.