Why Bubbles Coalesce Faster than Droplets: The Effects of Interface Mobility and Surface Charge.

Air bubbles in pure water appear to coalesce much faster compared to oil emulsion droplets at the same water solution conditions. The main factors explaining this difference in coalescence times could be interface mobility and/or pH-dependent surface charge at the water interface. To quantify the relative importance of these effects, we use high-speed imaging to monitor the coalescence of free-rising air bubbles with the water-air interface as well as free-falling fluorocarbon-oil emulsion droplets with a water-oil interface. We measure the coalescence times of such bubbles and droplets over a range of different water pH values (3.0, 5.6, 11.0). In the case of bubbles, a very fast coalescence (milliseconds) is observed for the entire pH range in pure water, consistent with the hydrodynamics of fully mobile interfaces. However, when the water-air interface is immobilized by the deposition of a monolayer of arachidic acid, the coalescence is significantly delayed. Furthermore, the coalescence times increase with increasing pH. In the case of fluorocarbon-oil droplets, the coalescence is always much slower (seconds) and consistent with immobile interface coalescence. The fluorocarbon droplet's coalescence time is also pH-dependent, with a complete stabilization (no coalescence) observed at pH 11. In the high electrolyte concentration, a 0.6 M NaCl water solution, bubbles, and droplets have similar coalescence times, which could be related to the bubble interface immobilization at the late stage of the coalescence process. Numerical simulations are used to evaluate the time scale of mobile and immobile interface film drainage.

Transferable Ga2O3 Membrane for Vertical and Flexible Electronics via One-Step Exfoliation.

Transferable Ga2O3 thin film membrane is desirable for vertical and flexible solar-blind photonics and high-power electronics applications. However, Ga2O3 epitaxially grown on rigid substrates such as sapphire, Si, and SiC hinders its exfoliation due to the strong covalent bond between Ga2O3 and substrates, determining its lateral device configuration and also hardly reaching the ever-increasing demand for wearable and foldable applications. Mica substrate, which has an atomic-level flat surface and high-temperature tolerance, could be a good candidate for the van der Waals (vdW) epitaxy of crystalline Ga2O3 membrane. Beyond that, benefiting from the weak vdW bond between Ga2O3 and mica substrate, in this work, the Ga2O3 membrane is exfoliated and transferred to arbitrary flexible and adhesive tape, allowing for the vertical and flexible electronic configuration. This straightforward exfoliation method is verified to be consistent and reproducible by the transfer and characterization of thick (∼380 nm)/thin (∼95 nm) κ-phase Ga2O3 and conductive n-type ß-Ga2O3. Vertical photodetectors are fabricated based on the exfoliated Ga2O3 membrane, denoting the peak response at ∼250 nm. Through the integration of Ti/Au Ohmic contact and Ni/Ag Schottky contact electrode, the vertical photodetector exhibits self-powered photodetection behavior with a responsivity of 17 mA/W under zero bias. The vdW-bond-assisted exfoliation of the Ga2O3 membrane demonstrated here could provide enormous opportunities in the pursuit of vertical and flexible Ga2O3 electronics.

Direct imaging of polymer filaments pulled from rebounding drops.

Polymer filaments form the foundation of biology from cell scaffolding to DNA. Their study and fabrication play an important role in a wide range of processes from tissue engineering to molecular machines. We present a simple method to deposit stretched polymer fibers between micro-pillars. This occurs when a polymeric drop impacts on and rebounds from an inclined superhydrophobic substrate. It wets the top of the pillars and pulls out liquid filaments which are stretched and can attach to adjacent pillars leaving minuscule threads, with the solvent evaporating to leave the exposed polymers. We use high-speed video at the microscale to characterize the most robust filament-forming configurations, by varying the impact velocity, substrate structure and inclination angle, as well as the PEO-polymer concentration. Impacts onto plant leaves or a randomized nano-structured surface leads to the formation of a branched structure, through filament mergers at the free surface of the drop. SEM shows the deposition of filament bundles which are thinner than those formed by evaporation or rolling drops. Raman spectroscopy identifies the native mode B stretched DNA filaments from aqueous-solution droplets.

Bubble eruptions in a multilayer Hele-Shaw flow.

We study the dynamical rearrangement of gravitationally unstable multilayer fluid inside the narrow vertical gap of a Hele-Shaw cell. Four layers of immiscible fluids are superposed inside the cell, which is subsequently turned over. We vary the fluid properties and the relative thicknesses of the layers. One of the layers is air, the others are immiscible liquids: olive oil, water-glycerin mixture, and perfluorohexane. The concentration of the glycerin-water mixture is used to vary its viscosity. We classify various different dynamics of stirring and breakthrough of adjacent layers. We note a prominent phenomenon, where an air finger breaks through the high-viscosity layer to erupt as a hemisphere into the lower-viscosity perfluorohexane layer above it. These eruptions have a periodic neck pinch-off accompanied with high-speed airflow which breaks up some of the low-viscosity liquid to eject a spray of fine droplets. We use high-speed video to characterize the details of the eruptions and how wetting, contact lines and three-dimensionalities play a key role. We also investigate the center-of-mass trajectories for each layer and notice counterflows, where the center of some layers can temporarily move against buoyancy. The top and bottom layers can interchange by channeling through the intermediate layers, which subsequently overturn on longer timescales. We also point out some unexpected dynamics occurring in the triple- and four-phase interactions. Specifically, droplet motions are as much affected by local viscosity as by the density gradients.

Interferometry and Simulation of the Thin Liquid Film between a Free-Rising Bubble and a Glass Substrate.

Because of their practical importance and complex underlying physics, the thin liquid films formed between colliding bubbles or droplets have long been the subject of experimental investigations and theoretical modeling. Here, we examine the possibility of accurately predicting the dynamics of the thin liquid film drainage using numerical simulations when compared to an experimental investigation of millimetric bubbles free-rising in pure water and colliding with a flat glass interface. A high-speed camera is used to track the bubble bounce trajectory, and a second high-speed camera together with a pulsed laser is used for interferometric determination of the shape and evolution of the thin liquid film profile during the bounce. The numerical simulations are conducted with the open source Gerris flow solver. The simulation reliability was first confirmed by comparison with the experimental bubble bounce trajectory and bubble shape evolution during the bounce. We further demonstrate that the simulation predicted time evolution for the shape of the thin liquid film profiles is in excellent agreement with the high-speed interferometry measured profiles for the entire experimentally accessible film size range. Finally, we discuss the implications of using numerical simulation together with theoretical modeling for resolving the complex processes of high velocity bubble and droplet collisions.

Cavitation upon low-speed solid-liquid impact.

When a solid object impacts on the surface of a liquid, extremely high pressure develops at the site of contact. Von Karman's study of this classical physics problem showed that the pressure on the bottom surface of the impacting body approaches infinity for flat impacts. Yet, in contrast to the high pressures found from experience and in previous studies, we show that a flat-bottomed cylinder impacting a pool of liquid can decrease the local pressure sufficiently to cavitate the liquid. Cavitation occurs because the liquid is slightly compressible and impact creates large pressure waves that reflect from the free surface to form negative pressure regions. We find that an impact velocity as low as ~3 m/s suffices to cavitate the liquid and propose a new cavitation number to predict cavitation onset in low-speed solid-liquid impact-scenarios. These findings imply that localized cavitation could occur in impacts such as boat slamming, cliff jumping, and ocean landing of spacecraft.

Spreading of Normal Liquid Helium Drops.

We have used video imaging and interferometric techniques to investigate the dynamics of spreading of drops of ^{4}He on a solid surface for temperatures ranging from 5.2 K (near the critical point) to 2.2 K (near T_{λ}). After an initial transient, the drops become pancake-shaped with a radius that grows as R(t)≈t^{α}, with α=0.149±0.002. The drops eventually begin to shrink due to evaporation driven by gravitational and curvature effects, which limits their lifetime to about 1000 s. Although helium completely wets the substrate, and the spreading takes place over a pre-existing adsorbed film, a distinct contact line with a contact angle of order one degree is visible throughout this process.

How drain flies manage to almost never get washed away.

Drain flies, Pshycoda spp. (Order Diptera, Family Psychodidae), commonly reside in our homes, annoying us in our bathrooms, kitchens, and laundry rooms. They like to stay near drains where they lay their eggs and feed on microorganisms and liquid carbohydrates found in the slime that builds up over time. Though they generally behave very sedately, they react quite quickly when threatened with water. A squirt from the sink induces them to fly away, seemingly unaffected, and flushing the toilet with flies inside does not necessarily whisk them down. We find that drain flies' remarkable ability to evade such potentially lethal threats does not stem primarily from an evolved behavioral response, but rather from a unique hair covering with a hierarchical roughness. This covering, that has never been previously explored, imparts superhydrophobicity against large droplets and pools and antiwetting properties against micron-sized droplets and condensation. We examine how this hair covering equips them to take advantage of the relevant fluid dynamics and flee water threats in domestic and natural environments including: millimetric-sized droplets, mist, waves, and pools of water. Our findings elucidate drain flies' astounding ability to cope with a wide range of water threats and almost never get washed down the drain.

A droplet reactor on a super-hydrophobic surface allows control and characterization of amyloid fibril growth.

Methods to produce protein amyloid fibrils, in vitro, and in situ structure characterization, are of primary importance in biology, medicine, and pharmacology. We first demonstrated the droplet on a super-hydrophobic substrate as the reactor to produce protein amyloid fibrils with real-time monitoring of the growth process by using combined light-sheet microscopy and thermal imaging. The molecular structures were characterized by Raman spectroscopy, X-ray diffraction and X-ray scattering. We demonstrated that the convective flow induced by the temperature gradient of the sample is the main driving force in the growth of well-ordered protein fibrils. Particular attention was devoted to PHF6 peptide and full-length Tau441 protein to form amyloid fibrils. By a combined experimental with the molecular dynamics simulations, the conformational polymorphism of these amyloid fibrils were characterized. The study provided a feasible procedure to optimize the amyloid fibrils formation and characterizations of other types of proteins in future studies.

Droplet impacts onto soft solids entrap more air.

We investigate the effects of surface stiffness on the air cushioning at the bottom of a liquid drop impacting onto a soft solid and the resulting entrapment of a central bubble. This was achieved using ultra-high-speed interferometry at 5 million frames per second and spatial resolution of 1.05 µm per pixel. The soft solid delays the effects of gas compressibility resulting in much larger air discs than corresponding impacts onto rigid surfaces. Using an effective impact velocity equal to half of the actual impact velocity brings the soft solid scaling behavior better in line with rigid substrate scaling. We also observe extended gliding of the drop as it initially avoids contact with the surface spreading over a thin layer of air and investigate the threshold velocity for the transition from gliding to ring contact. Such extended gliding layers have previously been seen for high-viscosity drop impacts, but not for low-viscosity liquids at the impact velocities used herein.

To Split or Not to Split: Dynamics of an Air Disk Formed under a Drop Impacting on a Pool.

When a drop falls and impacts on a liquid pool, it entraps an air disk below the drop, which then contracts into a central bubble. Here, we use high-speed imaging and high-resolution numerical simulations to characterize the air-disk contraction dynamics for different liquid properties. We show that the air disk can contract into a single central bubble, form a toroidal bubble, or split vertically into two smaller bubbles. We demonstrate that the transitions between the different regimes can be separated by an Ohnesorge number, Oh_{e}, based on the air-disk thickness. For the lowest Oh_{e}, we find a new regime, where vortex shedding from the rim of the contracting air disk breaks the vertical symmetry and prevents the bubble from splitting in two.

Free-Rising Bubbles Bounce More Strongly from Mobile than from Immobile Water-Air Interfaces.

Recently it was reported that the interface mobility of bubbles and emulsion droplets can have a dramatic effect not only on the characteristic coalescence times but also on the way that bubbles and droplets bounce back after collision (Vakarelski, I. U.; Yang, F.; Tian, Y. S.; Li, E. Q.; Chan D. Y. C.; Thoroddsen, S. T. Sci. Adv. 2019, 5, eaaw4292). Experiments with free-rising bubbles in a pure perfluorocarbon liquid showed that collisions involving mobile interfaces result in a stronger series of rebounds before the eventual rapid coalescence. Here we examine this effect for the case of pure water. We compare the bounce of millimeter-sized free-rising bubbles from a pure water-air interface with the bounce from a water-air interface on which a Langmuir monolayer of arachidic acid molecules has been deposited. The Langmuir monolayer surface concentration is kept low enough not to affect the water surface tension but high enough to fully immobilize the interface due to Marangoni stress effects. Bubbles were found to bounce much stronger (up to a factor of 1.8 increase in the rebounding distance) from the clean water interface compared to the water interface with the Langmuir monolayer. These experiments confirm that mobile surfaces enhance bouncing and at the same time demonstrate that the pure water-air interfaces behave as mobile fluid interfaces in our system. A complementary finding in our study is that the ethanol-air interface behaves as a robust mobile liquid interface. The experimental findings are supported by numerical simulations of the bubble bouncing from both mobile and immobile fluid interfaces.

Mobile-surface bubbles and droplets coalesce faster but bounce stronger.

Enhancing the hydrodynamic interfacial mobility of bubbles and droplets in multiphase systems is expected to reduce the characteristic coalescence times and thereby affect the stability of gas or liquid emulsions that are of wide industrial and biological importance. However, by comparing the controlled collision of bubbles or water droplets with mobile or immobile liquid interfaces, in a pure fluorocarbon liquid, we demonstrate that collisions involving mobile surfaces result in a significantly stronger series of rebounds before the rapid coalescence event. The stronger rebound is explained by the lower viscous dissipation during collisions involving mobile surfaces. We present direct numerical simulations to confirm that the observed rebound is enhanced with increased surface mobility. These observations require a reassessment of the role of surface mobility for controlling the dynamic stability of gas or liquid emulsion systems relevant to a wide range of processes, from microfluidics and pharmaceuticals to food and crude oil processing.

Stable-streamlined cavities following the impact of non-superhydrophobic spheres on water.

The formation of a stable-streamlined gas cavity following the impact of a heated Leidenfrost sphere on a liquid surface or a superhydrophobic sphere on water is a recently demonstrated phenomenon. A sphere encapsulated in a teardrop-shaped gas cavity was found to have near-zero hydrodynamic drag due to the self-adjusting streamlined shape and the free-slip boundary condition on the cavity interface. Here we show that such cavities can as well be formed following water impact from a sufficient height of non-superhydrophobic spheres with water contact angles between >30° and 120°. In this case the streamlined cavity is attached just above the sphere's equator, instead of entirely wrapping the sphere. Nevertheless, this sphere with attached cavity formation has near-zero drag and a predetermined free fall velocity in compliance with the Bernoulli law of potential flow. The effect of surfactant addition to the water solution is investigated. The shape and fall velocity of a sphere with streamlined cavity formation were unaffected by the addition of low surface modulus synthetic surfactants, but were destabilised when solutions containing high surface modulus surfactants, such as soaps, were used.

The air entrapment under a drop impacting on a nano-rough surface.

We study the impact of drops onto a flat surface with a nano-particle-based superhydrophobic coating, focusing on the earliest contact using 200 ns time-resolution. A central air-disc is entrapped when the drop impacts the surface, and when the roughness is appropriately accounted for, the height and radial extent of the air-disc follow the scaling laws established for impacts onto smooth surfaces. The roughness also modifies the first contact of the drop around the central air-disc, producing a thick band of micro-bubbles. The initial bubbles within this band coalesce and grow in size. We also infer the presence of an air-film residing inside the microstructure, at radial distances outside the central air-disc. This is manifest by the sudden appearance of microbubbles within a few microseconds after impact. The central air-disc remains pinned on the roughness, unless it is chemically altered to make it superhydrophilic.

Drag crisis moderation by thin air layers sustained on superhydrophobic spheres falling in water.

We investigate the effect of thin air layers naturally sustained on superhydrophobic surfaces on the terminal velocity and drag force of metallic spheres free falling in water. The surface of 20 mm to 60 mm steel or tungsten-carbide spheres is rendered superhydrophobic by a simple coating process that uses a commercially available hydrophobic agent. By comparing the free fall of unmodified spheres and superhydrophobic spheres in a 2.5 meter tall water tank, it is demonstrated that even a very thin air layer (∼1-2 µm) that covers the freshly dipped superhydrophobic sphere can reduce the drag force on the spheres by up to 80%, at Reynolds numbers from 105 to 3 × 105, owing to an early drag crisis transition. This study complements prior investigations on the drag reduction efficiency of model gas layers sustained on heated metal spheres falling in liquid by the Leidenfrost effect. The drag reduction effects are expected to have significant implications for the development of sustainable air-layer-based energy saving technologies.

Coalescence Dynamics of Mobile and Immobile Fluid Interfaces.

Coalescence dynamics between deformable bubbles and droplets can be dramatically affected by the mobility of the interfaces with fully tangentially mobile bubble-liquid or droplet-liquid interfaces expected to accelerate the coalescence by orders of magnitude. However, there is a lack of systematic experimental investigations that quantify this effect. By using high speed camera imaging we examine the free rise and coalescence of small air-bubbles (100 to 1300 µm in diameter) with a liquid interface. A perfluorocarbon liquid, PP11, is used as a model liquid to investigate coalescence dynamics between fully mobile and immobile deformable interfaces. The mobility of the bubble surface was determined by measuring the terminal rise velocity of small bubbles rising at Reynolds numbers, Re, less than 0.1 and the mobility of free PP11 surface by measuring the deceleration kinetics of the small bubble toward the interface. Induction or film drainage times of a bubble at the mobile PP11-air surface were found to be more than 2 orders of magnitude shorter compared to the case of bubble and an immobile PP11-water interface. A theoretical model is used to illustrate the effect of hydrodynamics and interfacial mobility on the induction time or film drainage time. The results of this study are expected to stimulate the development of a comprehensive theoretical model for coalescence dynamics between two fully or partially mobile fluid interfaces.

Double Contact During Drop Impact on a Solid Under Reduced Air Pressure.

Drops impacting on solid surfaces entrap small bubbles under their centers, owing to the lubrication pressure which builds up in the thin intervening air layer. We use ultrahigh-speed interference imaging, at 5 Mfps, to investigate how this air layer changes when the ambient air pressure is reduced below atmospheric. Both the radius and the thickness of the air disc become smaller with reduced air pressure. Furthermore, we find the radial extent of the air disc bifurcates, when the compressibility parameter exceeds ∼25. This bifurcation is also imprinted onto some of the impacts, as a double contact. In addition to the central air disc inside the first ring contact, this is immediately followed by a second ring contact, which entraps an outer toroidal strip of air, which contracts into a ring of bubbles. We find this occurs in a regime where Navier slip, due to rarefied gas effects, enhances the rate gas can escape from the path of the droplet.

Self-determined shapes and velocities of giant near-zero drag gas cavities.

Minimizing the retarding force on a solid moving in liquid is the canonical problem in the quest for energy saving by friction and drag reduction. For an ideal object that cannot sustain any shear stress on its surface, theory predicts that drag force will fall to zero as its speed becomes large. However, experimental verification of this prediction has been challenging. We report the construction of a class of self-determined streamlined structures with this free-slip surface, made up of a teardrop-shaped giant gas cavity that completely encloses a metal sphere. This stable gas cavity is formed around the sphere as it plunges at a sufficiently high speed into the liquid in a deep tank, provided that the sphere is either heated initially to above the Leidenfrost temperature of the liquid or rendered superhydrophobic in water at room temperature. These sphere-in-cavity structures have residual drag coefficients that are typically less than [Formula: see text] those of solid objects of the same dimensions, which indicates that they experienced very small drag forces. The self-determined shapes of the gas cavities are shown to be consistent with the Bernoulli equation of potential flow applied on the cavity surface. The cavity fall velocity is not arbitrary but is uniquely predicted by the sphere density and cavity volume, so larger cavities have higher characteristic velocities.