A standing Leidenfrost drop with Sufi whirling.

When a water drop is placed on a hot solid surface, it either undergoes explosive contact boiling or exhibits a stable state. In the latter case, the drop floats over an insulating layer of vapor generated by rapid vaporization of water at the surface/drop interface; this is known as the Leidenfrost state. Here, we discuss a previously unrecognized steady state in which a water drop "stands" on a hot smooth surface. In this state, the drop stabilizes itself with partial adhesion on the hot surface, leading to unique deformation and rotation behavior reminiscent of Sufi whirling-a form of spinning dance. Our analysis of this standing Leidenfrost state reveals the underlying mechanisms that drive the drop's stable partial adhesion and subsequent deformation with rotation. The heat-transfer efficiency of this standing state is up to 390% greater than that of the traditional floating Leidenfrost state.

Surface Nanostructures Promote Droplet Splash on Hot Substrates.

Splash, one of the most visually apparent droplet dynamics, can manifest on any surface above a certain impact velocity, regardless of surface wettability. Previous studies demonstrate that elevating the substrate temperature can suppress droplet splash, which is unfavorable for many practical applications, such as spray cooling and combustion. Here, we report that the suppression effect of substrate temperature on splash is nullified by utilizing surfaces with nanostructures. By manipulating air evacuation time through surface nanostructures, we have identified a pathway for precise control over the splash threshold and the ability to tailor the dependence of the splash onset on surface temperature. We further propose a theoretical criterion to determine different splash regimes by considering the competition between air evacuation and the development of flow instabilities. Our findings underscore the crucial role of nanostructures in splash dynamics, offering valuable insights for the control of splash in various industrial scenarios.

Surface-Charge-Assisted Microdroplet Generation on a Superhydrophobic Surface.

The ability to generate and manipulate droplets down to microscales has attracted great attention in a variety of applications, such as in printing, microreactors, and biological assays. However, the production of microdroplets is often limited by special equipment or the size of needles. Here, an unexplored and facile approach is demonstrated; microdroplets can be generated and trapped yet not pinned on a micro-nano-structured superhydrophobic surface by controllable surface charge during drop impact. Tiny droplets with a size at a scale of tens of microns to millimeters are generated by simply changing the impacting velocity, the size of the impact drop, or impact frequency. Theoretical analysis suggests the generation of the microdroplet as a result of the surface-charge-regulated adhesion, competing with liquid dynamic and interfacial energy. The distribution of surface charge which determines the size and the location of the microdroplet is at the top of the micro-nano-structured surface and dependent on the pressure field applied on the surface during the drop impact. The mobility of the resulting microdroplet that can be easily manipulated without liquid retention is also shown, by taking advantage of the shielding property of the surface charge. This facile yet effective method provides a promising candidate for the realization of tiny droplet-generating and -manipulating applications.

Dynamic wetting of Newtonian and viscoelastic fluids on microstructured surfaces.

Hypothesis Although extensive research has been conducted on the dynamic wetting of Newtonian fluids, limited insights have been gained for viscoelastic fluids, particularly on engineered surfaces. We hypothesize that differences in dynamic wetting on microstructured surfaces exist between such fluids, which may be attributed to variations in viscosity and elasticity as well as changes in the microscopic morphology of the moving contact line. Experiments To systematically investigate the wetting differences between Newtonian and viscoelastic fluids on microstructured surfaces, we conducted forced wetting experiments of glycerol-water and carboxymethyl cellulose aqueous solutions on microstructured polytetrafluoroethylene surfaces through a modified Wilhelmy plate method. Findings Results demonstrated an apparent difference in the relationship between the dynamic contact angle and moving velocity with different microstructured surfaces for Newtonian and viscoelastic fluids. The power-law exponent between the capillary number and cubic of the dynamic contact angle increases with the strengthening of shear thinning and elastic effects. In contrast, this exponent is rarely influenced by the scale of microstructured surfaces, particularly in highly viscous regions where viscous force dominates. In addition, viscosity affects the viscous bending and distance that liquid molecules jump at the contact line. These findings have potential applications in coating complex fluids on engineered surfaces.

Aerodynamic Super-Repellent Surfaces.

Repelling liquid drops from engineering surfaces has attracted great attention in a variety of applications. To achieve efficient liquid shedding, delicate surface textures are often introduced to sustain air pockets at the liquid-solid interface. However, those surfaces are prone to suffer from mechanical failure, which may bring reliability issues and thus limits their applications. Here, inspired by the aerodynamic Leidenfrost effect, we present that impacting drops are directionally repelled from smooth surfaces supplied with an exogenous air layer. Our theoretical analysis reveals that the synchronized nonwetting and oblique bouncing behavior is attributed to the aerodynamic force arising from the air layer. The versatility and practicability of our approach allow for drop repellency without the aid of any surface wettability treatment and also avoid the consideration of mechanical stability issues, which thereby provides a promising candidate for the applications that necessitate liquid shedding, e.g., resolve the problem of tiny raindrop adhesion on the automobile side window during driving.

Liquid-Pressure-Guided Superhydrophobic Surfaces with Adaptive Adhesion and Stability.

The dynamic application environments of superhydrophobic surfaces, such as in the manufacturing, chemical, and garment industries, require the fast adaptiveness of the surfaces to their surroundings. Despite the progress in materials and structural design of superhydrophobic surfaces, simultaneously achieving high superhydrophobic stability and low adhesion by traditional design is still challenging. Here, a liquid-pressure-guided superhydrophobic surface with self-adjustable solid-liquid stability, and adhesion is demonstrated when reacting to the dynamic environmental requirements. To understand the underlying adaptive processes, the liquid impalement dynamics is imaged in three dimensions by confocal microscopy and the stability and superhydrophobicity are measured in varied systems. It is envisioned that the design strategy of liquid-pressure-guided superhydrophobic surfaces with dynamic anti-infiltration capability can stimulate the development of stable superhydrophobicity under complex wetting conditions.

Charge Density Gradient Propelled Ultrafast Sweeping Removal of Dropwise Condensates.

Continuous sweeping of dropwise condensates is an effective form of vapor to liquid transition in terms of thermal transport at a solid/liquid interface. However, using conventional approaches, it is difficult to simultaneously achieve small activating size and fast departure of condensed droplets with high efficiency, due to the insufficient driving force compared to adhesion. Here, we propose an unexplored method to stimulate a frequent sweeping removal of dropwise condensates at ultrahigh efficiency on a superhydrophobic substrate, aided by a charge density gradient (CDG). We show that the CDG can be injected inside a superhydrophobic substrate on which the condensate droplet with jump-induced charges starts to sweep at a small size down to the microscale followed by quick snowball-like growing and chase-like propelling. The incorporation of the CDG on a superhydrophobic substrate enables a continuous, fast, frequent, long-range, and gravity-independent droplet removal during condensation, making this strategy a promising solution for diverse applications in water harvesting, antifogging, and anti-icing.

Density Maximization of One-Step Electrodeposited Copper Nanocones and Dropwise Condensation Heat-Transfer Performance Evaluation.

Currently, it is still a great challenge to obtain copper-based high-efficient dropwise condensation heat transfer (CHT) interfaces via template-free electrodepositing technologies. Here, we report that the density of template-free electrodeposited copper nanocones can maximally reach 1.5 × 106/mm2 by the synergistic control of substrate surface roughness, poly(ethylene glycol) (PEG) molecular weight, and PEG concentration. After thiol modification, the densely packed copper nanocone samples can present low-adhesive superhydrophobicity and condensate microdrop self-jumping function at ambient environment. Condensation heat and mass transfer characterizations show that the CHT coefficient of copper surfaces can maximally enhance 98% for 20 °C vapor and 51% for 40 °C vapor by in situ growth of superhydrophobic densely packed copper nanocones. Although the dropwise condensation mode can change from the jumping mode to the mixed jumping and sweeping mode and the shedding-off mode along with the increase of surface subcooling and vapor temperature, the CHT performance of the nanosample is still superior to that of the contrast flat hydrophobic surface during the whole testing range of surface subcooling. As vapor temperature increases to 80 °C, the CHT performance of the nanosample is inferior to that of the contrast sample. The CHT enhancement under low-temperature vapor should be ascribed to the enhancement of small-drop mass transfer ability caused by low-adhesive superhydrophobicity nature of nanosample surfaces. Their performance degradation mainly results from increased drop-drop drag force along with the increase of surface subcooling and vapor temperature. In sharp contrast, the CHT deterioration under high-temperature vapor should be ascribed to larger drop-surface adhesion and drop-drop drag force. The former is caused by vapor penetration, whereas the latter is caused by the dramatically increased nucleation density and growth rate of condensates. These findings would help design and develop copper-based high-efficiency condensation heat transfer interfaces.

Prompting Splash Impact on Superamphiphobic Surfaces by Imposing a Viscous Part.

It is widely acknowledged that splash impact can be suppressed by increasing the viscosity of the impinging drop. In this work, however, by imposing a highly viscous drop to a low-viscosity drop, it is demonstrated that the splash of the low-viscosity part of this Janus drop on superamphiphobic surfaces can be significantly promoted. The underlying mechanism is that the viscous stress exerted by the low-viscosity component drives the viscous component moving in the opposite direction, enhancing the spreading of the low-viscosity side and thereby its rim instability. The threshold velocity, above which splashing occurs, can be tuned by varying the viscosity ratio of the Janus drop. Moreover, the impact of the Janus drop can be employed to verify the mechanism of splash.