Enhancement of boiling heat transfer on biphilic (mixed-wettability) surfaces faces a sudden reversal at low pressures, which is brought about by excessive contact-line spreading across the wetting heterogeneities. We employ the diffuse-interface approach to numerically study bubble expansion on a heating surface that consists of opposing wettabilities. The results show a dramatic shift in the dynamics of a traversing contact line across the wettability divide under different gravities, which correspond to variable bubble growth rates. Specifically, it is found that the contact-line propagation tends to follow closely the rapidly expanding bubble at low gravity, with only a brief interruption at the border between the hydrophobic and hydrophilic sections of the surface. Only when the bubble growth becomes sufficiently weakened at high gravity does the contact line get slowed down drastically to the point of being nearly immobilized at the edge of the hydrophilic surface. The following bubble expansion, which faces strong limitations in the direction parallel to the surface, features a consistent apparent contact angle at around 66.4 degrees, regardless of the wettability combination. A simple theoretical model based on the force-balance analysis is proposed to describe the physical mechanism behind such a dramatic transition in the contact-line behavior.

Microstructured surfaces that control the direction of liquid transport are not only ubiquitous in nature, but they are also central to technological processes such as fog/water harvesting, oil–water separation, and surface lubrication. However, a fundamental understanding of the initial wetting dynamics of liquids spreading on such surfaces is lacking. Here, we show that three regimes govern microstructured surface wetting on short time scales: spread, stick, and contact line leaping. The latter involves establishing a new contact line downstream of the wetting front as the liquid leaps over specific sections of the solid surface. Experimental and numerical investigations reveal how different regimes emerge in different flow directions during wetting of periodic asymmetrically microstructured surfaces. These insights improve our understanding of rapid wetting in droplet impact, splashing, and wetting of vibrating surfaces and may contribute to advances in designing structured surfaces for the mentioned applications.

In this paper, we numerically study pool boiling of a binary (water and nitrogen) mixture on a surface endowed with a combination of hydrophobicity and hydrophilicity (i.e., the so called biphilic surface). Here we adopt a numerical approach based on the phase field theory, where the vapor-liquid interface is assumed to be of a finite thickness (hence diffusive in nature) and requires no explicit tracking schemes. The theoretical modeling of two-phase heat and mass transfer in water diluted with nitrogen demonstrates the signiant impact of impurities on bubble dynamics. The simulations show that locally high concentrations of nitrogen gas within the vapor bubble is essential to weakening the condensation effect, which results in sustained bubble growth and ultimately (partial) departure from the surface under the artificially enlarged gravity. Simply increasing the solubility of nitrogen in water, however, turns out to be counterproductive because possible re-dissolution of the aggregated nitrogen by the bulk water could deprive the bubble of vital gas contents, leading instead to continuous bubble shrinkage and collapse. Additionally, it is found that with the significant accumulation of nitrogen, the bubble interface is increasingly dominated by a strong interfacial thermocapillary flow due to the Marangoni effect.

The heat and mass transfer of deformable droplets in turbulent flows is crucial. to a wide range of applications, such as cloud dynamics and internal combustion engines. This study investigates a single droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. Phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Droplet deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables are quantified and averaged over both the liquid and vapour phases. The occurrence of evaporation and condensation is correlated to temperature fluctuations, surface tension variation and turbulence intensity. The temporal spectra of droplet deformations are analysed and related to the droplet surface area. Different modes of oscillation are clearly identified from the deformation power spectrum for low Taylor Reynolds number Re, whereas nonlinearities are produced with the increase of Re A, as intermediate frequencies are seen to overlap. As an outcome, a continuous spectrum is observed, which shows a decrease in the power spectrum that scales as similar to f(-3) Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also developed.

For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures-even below the boiling point-is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.

Dynamic wetting problems are fundamental to understanding the interaction between liquids and solids. Even in a superficially simple experimental situation, such as a droplet spreading over a dry surface, the result may depend not only on the liquid properties but also strongly on the substrate-surface properties; even for macroscopically smooth surfaces, the microscopic geometrical roughness can be important. In addition, because surfaces may often be naturally charged or electric fields are used to manipulate fluids, electric effects are crucial components that influence wetting phenomena. We investigate the interplay between electric forces and surface structures in dynamic wetting. Although surfacemicrostructures can significantly hinder spreading, we find that electrostatics can " cloak" themicrostructures, that is, deactivate the hindering. We identify the physics in terms of reduction in contact-line friction, which makes the dynamic wetting inertial force dominant and insensitive to the substrate properties.

We conduct numerical experiments on polymeric droplets hitting a at solid surface, inrealistic circumstances. The results present time-resolved non-Newtonian stresses actingin the droplet, which so far have not been measured experimentally. Comparing withthe simulation of the impact of a Newtonian droplet, the eects of viscoelasticity ondroplet behaviors such as splashing, the maximum spreading diameter and deformationare analyzed. With detailed information on the contact region, we demonstrate how thecontact line behaves according to the transition of uid from elasticity dominated toshear-thinning dominated when a droplet expands and contracts on the substrate. Thework is inspired by several experiments on similar cases and we discuss whether and howthe elasticity in an impinging droplet takes eect.

The effect of gasification on the dynamics and kinematics of immersed spherical and non-spherical solid particles have been investigated using the three-dimensional lattice Boltzmann method. The gasification was performed by applying mass injection on particle surface for three cases: flow passing by a fixed sphere, rotating ellipsoid in simple shear flow, and a settling single sphere in a rectangular domain. In addition, we have compared the accuracy of employing two different fluid-solid interaction methods for the particle boundary. The validity of the gasification model was studied by comparing computed the mass flux from the simulation and the calculated value on the surface of the particle. The result was used to select a suitable boundary method in the simulations combined with gasification. Moreover, the reduction effect of the ejected mass flux on the drag coefficient of the fixed sphere have been validated against previous studies. In the case of rotating ellipsoid in simple shear flow with mass injection, a decrease on the rate of rotation was observed. The terminal (maximum) velocity of the settling sphere was increased by increasing the ratio of radial flux from the particle boundary.

The motion of a single ellipsoidal particle in simple shear flow can provide valuable insights toward understanding suspension flows with nonspherical particles. Previously, extensive studies have been performed on the ellipsoidal particle with rotational symmetry, a so-called spheroid. The nearly prolate ellipsoid (one major and two minor axes of almost equal size) is known to perform quasiperiodic or even chaotic orbits in the absence of inertia. With small particle inertia, the particle is also known to drift toward this irregular motion. However, it is not previously understood what effects from fluid inertia could be, which is of highest importance for particles close to neutral buoyancy. Here, we find that fluid inertia is acting strongly to suppress the chaotic motion and only very weak fluid inertia is sufficient to stabilize a rotation around themiddle axis. Themechanism responsible for this transition is believed to be centrifugal forces acting on fluid, which is dragged along with the rotational motion of the particle. With moderate fluid inertia, it is found that nearly prolate triaxial particles behave similarly to the perfectly spheroidal particles. Finally, we also are able to provide predictions about the stable rotational states for the general triaxial ellipsoid in simple shear with weak inertia.