Showing papers on "Laplace pressure published in 2019"

Is Contact Nucleation Caused by Pressure Perturbation

Abstract: The reason why ice nucleation is more efficient by contact nucleation than by immersion nucleation has been elusive for over half a century. Six proposed mechanisms are summarized in this study. Among them, the pressure perturbation hypothesis, which arose from recent experiments, can qualitatively explain nearly all existing results relevant to contact nucleation. To explore the plausibility of this hypothesis in a more quantitative fashion and to guide future investigations, this study assessed the magnitude of pressure perturbation needed to cause contact nucleation and the associated spatial scales. The pressure perturbations needed were estimated using measured contact nucleation efficiencies for illite and kaolinite, obtained from previous experiments, and immersion freezing temperatures, obtained from well-established parameterizations. Pressure perturbations were obtained by assuming a constant pressure perturbation or a Gaussian distribution of the pressure perturbation. The magnitudes of the pressure perturbations needed were found to be physically reasonable, being achievable through possible mechanisms, including bubble formation and breakup, Laplace pressure arising from the distorted contact line, and shear. The pressure perturbation hypothesis provides a physically based and experimentally constrainable foundation for parameterizing contact nucleation that may be useful in future cloud-resolving models.

Bioinspired slippery cone for controllable manipulation of gas bubbles in low-surface-tension environment

Abstract: Manipulating bubbles in surfactant solutions or oil mediums is of vital importance in daily life and industries concerned with cosmetics, food, fermentation, mineral flotation, etc. However, realizing controllable regulation of a bubble's behavior is quite challenging in a low-surface-tension aqueous environment, which is mainly attributed to the strong affinity of liquid molecules to a solid surface to prevent the efficient interaction of gas bubbles with the solid surface. To address these issues, herein, we have taken inspiration from cactus spines and pitcher plants to develop a slippery copper cone (SCC), which can facilely manipulate gas bubble in surfactant solutions (as low as ∼29.9 mN/m, 20 °C), e. g., directional and continuous transportation of gas bubbles. This intriguing capability mainly originates from the cooperation of the conical morphology engendering a Laplace pressure and the slippery surface with low friction force but high affinity to bubbles. In addition, the SCC also shows an elegant capability of transporting gas bubbles in various organic solvents, such as formamide (57.4 mN/m, 20 °C), glycol (46.5 mN/m, 20 °C), dibutyl phthalate (37.0 mN/m, 20 °C), and dimethylformamide (35.8 mN/m, 20 °C). Furthermore, the prepared SCC also demonstrated distinguished feasibility in antibuoyancy bubble delivery, efficient collection of acidic CO2 microbubbles, and the underwater reaction of hydrogen and oxygen, endowing it with promising applications in various complex low-surface-tension environments.

Roughness dependent wettability of sputtered copper thin films: The effect of the local surface slope

Abstract: Here, we investigated the static and the dynamic wetting behaviors of copper (Cu) thin films deposited by DC magnetron sputtering. The deposited films have random rough surfaces for which the rms roughness amplitude σ, the lateral correlation length ξ, and the roughness exponent α were obtained from the analysis of height topography images acquired by atomic force microscopy. The time-dependent height-height correlation functions indicated anomalous kinetic roughening with roughness exponents α ≈ 0.9 and evolving roughness parameters σ and ξ with deposition time. The latter yields a nonstationary local surface slope σ / ξ that has a crucial impact on the surface wettability. Indeed, static and dynamic contact angles’ (CAs) measurements revealed two wetting regimes associated with different growth stages leading to a transition from a metastable Cassie-Baxter to a Wenzel-like state for the roughest films. Moreover, the increasing roughness with well distributed peaks and valleys leads to increasing CAs due to trapped air in surface cavities, while after some point the larger surface features lead to a decrement of the CAs that vary only slightly with further roughening. Although the apparent wetting transition with increasing surface roughness is not favored by the local Laplace pressure estimation, the energy of the system decreases with surface roughening, or equivalently increasing local surface slope, favoring energetically a Wenzel state. Under these conditions, the water droplet can spontaneously fill the surface cavities once the impregnation is initiated by the hydrophilic nature of the surface, in agreement with our experiments for significantly large local surface slopes ρ (>0.1) and large roughness exponents α ∼ 1.

Surface tension and the origin of the circular hydraulic jump in a thin liquid film

Abstract: It was recently claimed by Bhagat et al. (J. Fluid Mech. vol. 851 (2018), R5) that the scientific literature on the circular hydraulic jump in a thin liquid film is flawed by improper treatment and severe underestimation of the influence of surface tension. Bhagat {\em et al.} use an energy equation with a new surface energy term that is introduced without reference, and they conclude that the location of the hydraulic jump is determined by surface tension alone. We show that this approach is incorrect and derive a corrected energy equation. Proper treatment of surface tension in thin film flows is of general interest beyond hydraulic jumps, and we show that the effect of surface tension is fully contained in the Laplace pressure due to the curvature of the surface. Following the same approach as Bhagat et al., i.e., keeping only the first derivative of the surface velocity, the influence of surface tension is, for thin films, much smaller than claimed by them. We further describe the influence of viscosity in thin film flows, and we conclude by discussing the distinction between time-dependent and stationary hydraulic jumps.

Bioinspired triangular patterns for water collection from fog.

Abstract: Cacti use spines with conical geometry to transport water to its base. A conical shape with curvature gradient generates a Laplace pressure gradient along the droplet, which is responsible for droplet motion. In this study, the triangular shape was used which also generates a Laplace pressure gradient along the droplet. A bioinspired surface, composed of a hydrophilic triangular pattern surrounded by a rim of superhydrophobic region, was used to transport water collected from the fog on the hydrophilic pattern. The growing droplets start to coalesce into bigger ones. Eventually, they are big enough to touch the superhydrophobic borders, which trigger the transport motion. Droplet mobility and water collection measurements were made on triangular patterns with various geometries to determine the most efficient configurations. Results from this study can be used to enhance the performance of water collection systems from fog. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Resistant energy analysis of self-pulling process during dropwise condensation on superhydrophobic surfaces

Abstract: Recently the development of superhydrophobic surfaces with one-tier or hierarchical textures has drawn increasing attention because enhanced condensation heat transfer has been observed on such biomimetic surfaces in well-tailored supersaturation or subcooling conditions. However, the physical mechanisms underlying condensation enhancement are still less understood. Here we report an energy-based analysis on the formation and growth of condensate droplets on two-tier superhydrophobic surfaces, which are fabricated by decorating carbon nanotubes (CNTs) onto microscale fluorinated pillars. Thus-formed hierarchical surfaces with two tier micro/nanoscale roughness are proved to be superior to smooth surfaces in the spatial control of condensate droplets. In particular, we focus on the self-pulling process of condensates in the partially wetting morphology (PW) from surface cavities due to intrinsic Laplace pressure gradient. In this analysis, the self-pulling process of condensate tails is resisted by adhesion energy, viscous dissipation, contact line dissipation and line tension in a combined manner. This process can be facilitated by adjusting the configuration and length scale of the first-tier texture. The optimum design can not only lower the total resistant energy but also favor the out-of-plane motion of condensate droplets anchored in the first-tier cavity. It is also shown that engineered surface with hierarchical roughness is beneficial to remarkably mitigating contact line dissipation from the perspective of molecular kinetic theory (MKT). Our study suggests that scaling down surface roughness to submicron scale can facilitate the self-propelled removal of condensate droplets.

Stability of Evaporating Droplets on Chemically Patterned Surfaces.

Abstract: The stability of water droplets on striped surfaces exposing regions of different wettability is studied experimentally, numerically, and based on a scaling model. Different values of the stripe widths and different contact angle contrasts between the hydrophilic and hydrophobic stripes are considered. The boundary between the contact angle contrasts leaving the droplets intact and those leading to droplet breakup is computed numerically. The minimum contrast for which breakup occurs increases with increasing hydrophobic contact angle. The existence of an unstable and a stable regime is confirmed experimentally. In the unstable regime, when approching droplet breakup, a configuration with two liquid fingers on the hydrophilic stripes connected by a capillary bridge on the hydrophobic stripe is found. For decreasing volumes, the width of this capillary bridge decreases until a critical value is reached at which the droplet breaks up. The critical width depends on the ratio of the hydrophilic and the hydrophobic stripe width. A simple scaling model is presented with which the critical width can be predicted. According to the model, the droplet becomes unstable when the increasing Laplace pressure inside the bridge can no longer be balanced by the pressure inside the liquid fingers on the hydrophilic stripes.

Surface tension of supercooled water nanodroplets from computer simulations.

Abstract: We estimate the liquid-vapor surface tension from simulations of TIP4P/2005 water nanodroplets of size N = 100 to 2880 molecules over a temperature T range of 180 K-300 K. We compute the planar surface tension γp, the curvature-dependent surface tension γs, and the Tolman length δ, via two approaches, one based on the pressure tensor (the "mechanical route") and the other on the Laplace pressure (the "thermodynamic route"). We find that these two routes give different results for γp, γs, and δ although in all cases, we find that δ ≥ 0 and is independent of T. Nonetheless, the T dependence of γp is consistent between the two routes and with that of Vega and de Miguel [J. Chem. Phys. 126, 154707 (2007)] down to the crossing of the Widom line at 230 K for ambient pressure. Below 230 K, γp rises more rapidly on cooling than predicted from behavior for T ≥ 300 K. We show that the increase in γp at low T is correlated with the emergence of a well-structured random tetrahedral network in our nanodroplet cores and thus that the surface tension can be used as a probe to detect behavior associated with the proposed liquid-liquid phase transition in supercooled water.

Water droplet dynamics on bioinspired conical surfaces.

Abstract: Cacti use the Laplace pressure gradient due to conical geometry as a mechanism for collecting water from fog. Bioinspired surfaces using conical geometry can be developed for water collection from fog for human consumption. A systematic study is presented which investigates the dynamics of water droplets on a bioinspired conical surface. A series of experiments was conducted where a known volume of droplets was deposited on the cone. This was followed by an investigation into droplet dynamics where the droplets are deposited from fog and the volume is unknown. This includes a study on the macroscopic level as well as the microscopic level. The main parameters that were varied for these tests were the tip angle and the cone orientation. The droplet movement observed was compared relatively. Based on captured videos of droplet movement, distance travelled and velocities were measured. The Laplace pressure gradient, gravity and droplet coalescence were found to be the mechanisms of droplet movement on a conical surface. The findings of this study should be of interest in designing bioinspired surfaces with high water collection. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Molecular Simulation of Sorption-Induced Deformation in Atomistic Nanoporous Materials.

Abstract: An atomistic slit pore model is built to study the sorption-induced deformation of nanoporous materials with the help of molecular simulation. Both sorption and strain isotherms are determined to probe the anisotropic deformation behavior induced upon molecular adsorption. A detailed analysis shows that the driving microscopic mechanisms at different sorption stages are different. At high relative pressure, as expected from the classical macroscopic picture, the pore deformation is governed by the Laplace pressure as the pore gets filled with liquid because of capillary condensation. In such situation, the strain in normal and longitudinal directions can be predicted from the stiffness modulus in the corresponding direction. At low pressure, when liquid films are adsorbed at the pore surfaces and separated by the vapor phase, the strain is driven by the attractive solid–fluid forces and in-plane pressure within the film, and the deformation is confined in the direction parallel to the film–solid interface...

Bioinspired conical design for efficient water collection from fog

Abstract: Nature is known for using conical shapes to transport the collected water from fog for consumption or storage. The curvature gradient of the conical shape creates a Laplace pressure gradient in the water droplets which drives them towards the region of lower curvature. Linear cones with linearly increasing radii have been studied extensively. A smaller tip angle cone transports water droplets farther because of higher Laplace pressure gradient. Whereas a larger tip angle with a larger surface slope transports water droplets because of higher gravitational forces. In this study, for the first time, a nonlinear cone with a concave profile has been designed with small tip angle and nonlinearly increasing radius to maximize water collection. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Interaction of Particles with Surfactant Thin Films: Implications for Dust Suppression.

Abstract: Understanding the interaction of particles with foams is important in antifoaming applications and dust suppression. In the former, the aim is for the particles to break the foam, whereas in the latter it is desirable that the stability of the foam is maintained or enhanced. The interaction of particles of different wettabilities with thin surfactant films is investigated with a Sheludko cell, enabling the thinning and rupture of the films to be studied in the presence and absence of a particle, using white-light interferometry. The films were prepared from the surfactant cetyltrimethylammonium bromide and a commercial dust suppression foaming agent. The film lifetimes are extended upon the addition of hydrophilic particles and reduced upon the addition of hydrophobic particles with advancing contact angles >90°. The Laplace pressure in the film surrounding a particle is calculated as a function of the contact angle and particle size, revealing that the meniscus surrounding hydrophilic particles has a positive Laplace pressure, which increases the lifetime of the film.

Acoustic Bubble Suppression by Constructing a Hydrophilic Coating on HDPE Surface.

Abstract: Ultrasonic bubbles on the solid surface of various sonochemical devices largely affect signal resolution due to the serious reflection/scattering of sound waves. The Laplace pressure of the cavitat...

Multicomponent flow on curved surfaces: A vielbein lattice Boltzmann approach

Abstract: We develop and implement a novel finite difference lattice Boltzmann scheme to study multicomponent flows on curved surfaces, coupling the continuity and Navier-Stokes equations with the Cahn-Hilliard equation to track the evolution of the binary fluid interfaces. The standard lattice Boltzmann method relies on regular Cartesian grids, which makes it generally unsuitable to study flow problems on curved surfaces. To alleviate this limitation, we use a vielbein formalism to write down the Boltzmann equation on an arbitrary geometry, and solve the evolution of the fluid distribution functions using a finite difference method. Focussing on the torus geometry as an example of a curved surface, we demonstrate drift motions of fluid droplets and stripes embedded on the surface of such geometries. Interestingly, they migrate in opposite directions: fluid droplets to the outer side while fluid stripes to the inner side of the torus. For the latter we demonstrate that the global minimum configuration is unique for small stripe widths, but it becomes bistable for large stripe widths. Our simulations are also in agreement with analytical predictions for the Laplace pressure of the fluid stripes, and their damped oscillatory motion as they approach equilibrium configurations, capturing the corresponding decay timescale and oscillation frequency. Finally, we simulate the coarsening dynamics of phase separating binary fluids in the hydrodynamics and diffusive regimes for tori of various shapes, and compare the results against those for a flat two-dimensional surface. Our finite difference lattice Boltzmann scheme can be extended to other surfaces and coupled to other dynamical equations, opening up a vast range of applications involving complex flows on curved geometries.

Aggregation and coalescence of partially crystalline emulsion drops investigated using optical tweezers.

Abstract: The solid content of viscoelastic emulsion drops is known to affect their propensity for aggregation and their subsequent coalescence behaviour, where the balance between the drive to reduce surface tension and the straining of an internal viscoelastic network is able to create a plethora of stable partially-coalesced states. The latter has previously been elegantly demonstrated in synthetic systems, generated using oil containing different phase volumes of added solids, with micro-pipette experiments carried out on emulsion drops of several tens of microns in size. Herein we carry out experiments in the same spirit but aided by optical tweezers (OT) and using smaller micron-sized emulsion drops generated from milk fat. Given the size dependence of Brownian fluctuations and Laplace pressure the experimental investigation of these smaller drops is not necessarily a trivial extension of the previous work. The solid content of initially separated drops is controlled using a temperature-cycling regime in the sample preparation protocol, and subsequently the propensity for drops to remain joined or not after being brought into contact was examined. Aggregated pairs of drops were then subjected to an increase in temperature, either locally using a high-powered laser, or more globally using a custom-made Peltier temperature-controller. By heating to different degrees, the amount of fat crystals in the drops was able to be controlled, with progressively more compact partially-coalesced states, and eventually complete coalescence generated as the solid content was reduced. While in contrast to previous studies, the emulsion studied here was quite different in size and nature, and the solid content was controlled using temperature, the same underlying physics was nevertheless observed.

Magnetically stimulating capillary effect for reversible wet adhesions.

Abstract: Despite fascinating natural examples of switchable adhesives to wet surfaces, strategies for an artificially switching capillary adhesion system in situ remains a challenge. Here, we develop a smart reversible magnetic fluid (MF) meniscus adhesion system whose capillary effect can be regulated by external magnetic stimuli. It is revealed that the MF filled joint between two solid surfaces undergoes alteration of its adhesive properties in response to the external stimulus of a varying magnetic field. Compared with the original capillary force (without stimuli), the stimulated one increases or decreases depending on the distributions of applied magnetic field intensities, allowing for switchable adhesive behavior. In addition to the Laplace pressure, hydrostatic pressure induced by the intensity difference in the magnetic field between the inner and outer surfaces of the meniscus was found to contribute to wet adhesion, which accounted for the reversibility. Theoretical models of reversible adhesions have been built and solved as well, and agree well with the experiment results. Our findings not only provide a deep understanding of MF capillary adhesion, but also provide a new method to design reversible wet adhesion systems.

Fabrication of pixelated liquid crystal nanostructures employing the contact line instabilities of droplets.

Abstract: A liquid crystal (LC) droplet resting on a poly-dimethylsiloxane substrate could rapidly spread upon solvent vapour annealing to form a non-uniform film. While the solvophobic surfaces restricted the spreading of the droplet to form a thicker film upon solvent annealing, the solvophilic substrates allowed the formation of a thinner film under similar conditions. Withdrawal of the solvent exposure caused rapid evaporation of the solvent molecules from the film, especially near the retracting contact-line to form microscale LC-droplets, which shrunk into nanoscopic ones after evaporation of the excess solvent. The thinner films on solvophilic surfaces allowed the formation of droplets with smaller size and periodicity as small as ∼100 nm and ∼200 nm, respectively. Furthermore, the use of a patterned substrate could impose a large-area ordering on the nanodroplets. A theoretical model for an evaporating film of LC-solution revealed that the spacing of nanodroplets could be decided by the interplay of stabilizing and destabilizing components of capillary force while van der Waals interaction played a supportive role when the film was ultrathin near the contact line. The micro/nanodroplets thus formed showed an anomalous oscillatory rotational motion originating from the difference in the Laplace pressure near contact lines under the influence of an external electric field. The application of the Lorenz force to these droplets showed translation and rotational motions followed by ejection of satellite droplets.

Open surface multifunctional droplet manipulation platform fabricated by micromilling

Abstract: Superhydrophobic surfaces patterned with hydrophilic groove structures have promising applications in microfluidics. In this work, hydrophilic groove patterns with stable wettability were created directly on superhydrophobic Al surface by micromilling. Feasibility of applying diverse milled hydrophilic patterns, including dimples, ring grooves and reservoirs connected by winding channels in a variety of droplet manipulations, such as water/oil/air storage, water transport, etc. was tried out. Experimental results showed that the milled dimples and ring grooves on the superhydrophobic background can function as a multifunctional platform for storing micro-liter sized water, oil and gas bubbles; two milled reservoirs connected by a milled channel can serve as a open surface microfluidics to transport liquid using Laplace pressure difference. On the basis of the liquid transport between two reservoirs, more sophisticated liquid handling such as droplet separation and mixing were achieved by transporting liquid among multiple reservoirs. It is thus clear that the micromilling is a quite suitable approach to create complex hydrophilic groove structures on superhydrophobic background for applications to diverse interface-driven microfluidics.