Coalescence Cascade Research Articles

Condensations on electrowetting surfaces: An electrostatic liquid conveyor from dropwise-filmwise competition

In contrast to the traditional point of view, electrowetting creates a dynamic wetting regime. Hydrophilicity can penetrate or recede from the working surface –depending on the liquid locale rather than the substrate structures. Size regulation forms droplet clusters as condensation accumulates. Coalescence cascades and mass imbalances transfer the liquid from one zone to the next. What ensues is a passive electrostatic liquid conveyor that takes the condensation away from its constant self-stagnating cycles. Each working zone functions independently from one another according to its intended purpose: nucleation and diffusion, accumulation and collection. 40 to 50 percent of the increase in liquid recovery traces its root to the spatial-size regulation of the droplet population. Simultaneously, the gain from higher nucleations runs parabolically with electrostatic voltage. The competition between dropwise and filmwise tendencies determines the migration of condensates. Yet, the autonomy of the two means that nucleations are direct and unhindered as to how high an electrostatic voltage can be: the only limit is the material strength and endurance to sustain this concentration of electric potentials.

Mechanism of Qiwei Guibao Granules in treatment of premature ovarian failure based on proteomics

In this study, the underlying mechanism of Qiwei Guibao Granules(QWGB) in the treatment of premature ovarian fai-lure(POF) was explored by the proteomics technique. Firstly, the POF model was induced in mice by intragastric administration of Tripterygium wilfordii glycosides solution at 50 mg·kg~(-1) for 14 days. Ten days prior to the end of the modeling, the estrous cycle of mice was observed every day to evaluate the success of modeling. From the 1st day after modeling, the POF model mice were treated with QWGB by gavage every day and the treatment lasted four weeks. On the 2nd day after the end of the experiment, blood was collected from the eyeballs and the serum was separated by centrifugation. The ovaries and uterus were collected and the adipose tissues were carefully stripped. The organ indexes of the ovaries and uterus of each group were calculated. The serum estrogen(E_2) level of mice in each group was detected by ELISA. Protein samples were extracted from ovarian tissues of mice, and the differential proteins before and after QWGB intervention and before and after modeling were analyzed by quantitative proteomics using tandem mass tags(TMT). As revealed by the analysis of differential proteins, QWGB could regulate 26 differentially expressed proteins related to the POF model induced by T. wilfordii glycosides, including S100A4, STAR, adrenodoxin oxidoreductase, XAF1, and PBXIP1. GO enrichment results showed that the 26 differential proteins were mainly enriched in biological processes and cellular components. The results of KEGG enrichment showed that those differential proteins were involved in signaling pathways such as completion and coalescence cascades, focal adhesion, arginine biosynthesis, and terpenoid backbone biosynthesis. The complement and coalescence cascades signaling pathway was presumably the target pathway of QWGB in the treatment of POF. In this study, the proteomics technique was used to screen the differential proteins of QWGB in the treatment of POF in mice induced by T. wilfordii glycosides, and they were mainly involved in immune regulation, apoptosis regulation, complement and coagulation cascade reactions, cholesterol metabolism, and steroid hormone production, which may be the main mechanisms of QWGB in the treatment of POF.

Electric-field-mediated morpho-dynamic evolution in drop–drop coalescence phenomena in the inertio-capillary regime

When two drops collide, they may either exhibit complete coalescence or selectively generate secondary drops, depending on their relative sizes and physical properties, as dictated by a decisive interplay of the viscous, capillary, inertia and gravity effects. Electric field, however, is known to induce distinctive alterations in the topological evolution of the interfaces post-collision, by influencing a two-way nonlinear coupling between electro-mechanics and fluid flow as mediated by a topologically intriguing interfacial deformation. While prior studies primarily focused on the viscous-dominated regime of the resulting electro-coalescence dynamics, several non-intuitive features of the underlying morpho-dynamic evolution over the intertio-capillary regime have thus far remained unaddressed. In this study, we computationally investigate electrically modulated coalescence dynamics along with secondary drop formation mechanisms in the inertio-capillary regime, probing the interactions of two unequal-sized drops subjected to a uniform electric field. Our results bring out an explicit mapping between the observed topological evolution as a function of the respective initial sizes of the parent drops as well as their pertinent electro-physical property ratios. These findings establish electric-field-mediated exclusive controllability of the observed topological features, as well as the critical conditions leading to the transition from partial to complete coalescence phenomena. In a coalescence cascade, an electric field is further shown to orchestrate the numbers of successive stages of coalescence before complete collapse. However, an increase of the numbers of cascade stages with the electric field strength and parent droplet size ratio is non-perpetual, and the same is demonstrated to continue until only a threshold number of cascade stages is reached. These illustrations offer significant insights into leveraging the interplay of electrical, inertial and capillary-driven interactions for controllable drop manipulation via multi-drop interactions for a variety of applications ranging from chemical processing to emulsion technology.

Coalescence of bubbles in a high Reynolds number confined swarm

We investigate experimentally the coalescence cascade process for a confined swarm of deformable bubbles immersed in a bidimensional vertical cell filled with water. For different gas volume fractions, air bubbles of size $D_0$ larger than the cell thickness are injected at the bottom of the cell. The bubbles swarms transformation is explored using high-speed visualizations. The time evolution of each bubble in the swarm is determined using a specifically developed algorithm, enabling bubble tracking and coalescence detection. We determine the evolution of the bubble size distribution downstream from the injection point, and show that the stages of the coalescence cascade are characterized by the diameter, $D_{V90}$ , representative of the largest bubbles. The collision frequency of pairs of bubbles of sizes $D_k$ and $D_{k'}$ , $h(D_k, D_{k'})$ , and their coalescence efficiency, $\lambda$ , are obtained from the experiments. The efficiency is nearly constant, independently of the bubble sizes and of the gas volume fraction. Concerning collision frequency, our results reveal the existence of two different coalescence regimes depending on the capability of the bubbles to deform. Models describing $h(D_k, D_{k'})$ for both regimes are provided. They take into account the specific response of the bubble pair, which depends on the reduced diameter $D_p = 2 D_k D_{k'} / (D_k + D_{k'})$ , to the global swarm-induced agitation governed by $D_{V90}$ and the gas volume fraction. In the first regime, occurring for smaller $D_p$ , bubbles are brought together by agitation and rapidly coalesce, while for sufficiently large $D_p$ , both bubbles are able to deform and spend more time adapting mutually their shapes before coalescing.

From Hydrodynamics to Jet Quenching, Coalescence, and Hadron Cascade: A Coupled Approach to Solving the R_{AA}⊗v_{2} Puzzle.

Hydrodynamics and jet quenching are responsible for the elliptic flow v_{2} and suppression of large transverse momentum (p_{T}) hadrons, respectively, two of the most important phenomena leading to the discovery of a strongly coupled quark-gluon plasma in high-energy heavy-ion collisions. A consistent description of the hadron suppression factor R_{AA} and v_{2}, especially at intermediate p_{T}, however, remains a challenge. We solve this long-standing R_{AA}⊗v_{2} puzzle by including quark coalescence for hadronization and final state hadron cascade in the coupled linear Boltzmann transport-hydro model that combines concurrent jet transport and hydrodynamic evolution of the bulk medium. We illustrate that quark coalescence and hadron cascade, two keys to solving the puzzle, also lead to a splitting of v_{2} for pions, kaons, and protons in the intermediate p_{T} region. We demonstrate for the first time that experimental data on R_{AA}, v_{2}, and their hadron flavor dependence from low to intermediate and high p_{T} in high-energy heavy-ion collisions can be understood within this coupled framework.

Escape dynamics of liquid droplets confined between soft interfaces: non-inertial coalescence cascades.

We have studied the motion of liquid droplets located in the contact zone of two soft solids that are pressed together inside a liquid. The driving force of the motion is the gradient of elastic contact pressure. Upon reaching the perimeter of the contact spot, the droplets coalesce with the surrounding bulk liquid either in a continuous or discontinuous fashion. Droplets that exhibit complete coalescence move with a speed proportional to the pressure gradient along their trajectory. However, droplets that undergo partial coalescence cascades (up to 9 were observed) do not. The latter also move significantly slower. We have identified surface roughness as a potential mechanism causing the contrasting behavior.

Self-Sustained Cascading Coalescence in Surface Condensation.

Sustained dropwise condensation of water requires rapid shedding of condensed droplets from the surface. Here, we elucidate a microfluidic mechanism that spontaneously sweeps condensed microscale droplets without the need for the traditional droplet removal pathways such as use of superhydrophobicity for droplet rolling and jumping and utilization of wettability gradients for directional droplet transport among others. The mechanism involves self-generated, directional, cascading coalescence sequences of condensed microscale droplets along standard hydrophobic microgrooves. Each sequence appears like a spontaneous zipping process, can sweep droplets along the microgroove at speeds of up to ∼1 m/s, and can extend for lengths more than 100 times the microgroove width. We investigate this phenomenon through high-speed in situ microscale condensation observations and demonstrate that it is enabled by rapid oscillations of a condensate meniscus formed locally in a filled microgroove and pinned on its edges. Such oscillations are in turn spontaneously initiated by coalescence of an individual droplet growing on the ridge with the microgroove meniscus. We quantify the coalescence cascades by characterizing the size distribution of the swept droplets and propose a simple analytical model to explain the results. We also demonstrate that, as condensation proceeds on the hydrophobic microgrooved surface, the coalescence cascades recur spontaneously through repetitive dewetting of the microgrooves. Lastly, we identify surface design rules for consistent realization of the cascades. The hydrophobic microgrooved textures required for the activation of this mechanism can be realized through conventional, scalable surface fabrication methods on a broad range of materials (we demonstrate with aluminum and silicon), thus promising direct application in a host of phase-change processes.

Regular Flat Structure and Generalized Okubo System

We study a relationship between regular flat structures and generalized Okubo systems. We show that the space of variables of isomonodromic deformations of a regular generalized Okubo system can be equipped with a flat structure. As its consequence, we introduce flat structures on the spaces of independent variables of generic solutions to (classical) Painleve equations (except for PI). In our framework, the Painleve equations PVI–PII can be treated uniformly as just one system of differential equations called the four-dimensional extended WDVV equation. Then the well-known coalescence cascade of the Painleve equations corresponds to the degeneration scheme of the Jordan normal forms of a square matrix of rank four.

Breath Figures under Electrowetting: Electrically Controlled Evolution of Drop Condensation Patterns.

We show that electrowetting (EW) with structured electrodes significantly modifies the distribution of drops condensing onto flat hydrophobic surfaces by aligning the drops and by enhancing coalescence. Numerical calculations demonstrate that drop alignment and coalescence are governed by the drop-size-dependent electrostatic energy landscape that is imposed by the electrode pattern and the applied voltage. Such EW-controlled migration and coalescence of condensate drops significantly alter the statistical characteristics of the ensemble of droplets. The evolution of the drop size distribution displays self-similar characteristics that significantly deviate from classical breath figures on homogeneous surfaces once the electrically induced coalescence cascades set in beyond a certain critical drop size. The resulting reduced surface coverage, coupled with earlier drop shedding under EW, enhances the net heat transfer.

Influence of Marangoni stress on the variation in number of coalescence cascade stages

AbstractThe present work is an experimental and theoretical study on surfactant‐laden liquid drop impact on a liquid pool. When the drop breaks into a secondary droplet after impinging, then it is called partial coalescence. If this happens successively in self‐similar manner then it is called coalescence cascade. Three different types of surfactants, cationic, anionic, and non‐ionic, are used as drop fluid and water as the liquid pool. Here we report how the surfactant types and concentrations affect the number of stages in coalescence cascade. The experimental outcome revealed that the number of stages in cascade decreases with increasing surfactant concentration. Also, we determine that drop viscosity, density, and size play a crucial role while comparing the stages of cascade among three types of surfactants. We also perform scaling analysis to determine the contribution of inertial and surface forces in the cascade. A theoretical analysis using lubrication approximation has also been carried out to justify the experimental observations. The coalescence process is actually triggered by the drainage of entrapped air between the drop and pool. The theoretical analysis reveals that the faster air drainage rate and acceleration induces a strong Marangoni stress for necking and quick pinch off. Finally, it is shown that Marangoni flow, originated due to the surface tension difference between the drop and pool, is responsible for partial coalescence and a number of coalescence stages in cascade.

One‐Step Synthesis of Multifunctional Zinc‐Iron‐Oxide Hybrid Carbon Nanowires by Chemical Fusion for Supercapacitors and Interfacial Water Marbles

AbstractHybrid carbon nanowires with tunable elemental composition and nanoscale component mixing resolution can be synthesized by a chemical fusion method in one pot, using a unique heating mechanism of conducting polymers under the electromagnetic field of microwave. Carbon nanowire matrices hybridized with metals and oxides can be facilely synthesized by simple instrumentation and processes within tens of seconds. Different metallic precursors (e. g. Fe and Zn) can be selectively incorporated in the chemical fusion process that substantially integrated with the carbon nanowire matrices during the growth process while their characteristic functions are infused. The hybrid carbon nanowires show excellent electrochemical supercapacitive and supramolecular self‐assembly properties owing to the unique hybrid nanostructures and surface characteristics. “Interfacial water marbles” can be obtained by the supramolecular self‐assembly of hybrid carbon nanowires on the water/hexane interface. The interfacial water marbles are semi‐permanent and completely impede the event of coalescence cascade, demonstrating significant applications in mass transport, microfluidics, and interfacial synthesis.

A weighted multiple-relaxation-time lattice Boltzmann method for multiphase flows and its application to partial coalescence cascades

We present a lattice Boltzmann method (LBM) with a weighted multiple-relaxation-time (WMRT) collision model and an adaptive mesh refinement (AMR) algorithm for direct numerical simulation of two-phase flows in three dimensions. The proposed WMRT model enhances the numerical stability of the LBM for immiscible fluids at high density ratios, particularly on the D3Q27 lattice. The effectiveness and efficiency of the proposed WMRT–LBM–AMR is validated through simulations of (a) buoyancy-driven motion and deformation of a gas bubble rising in a viscous liquid; (b) the bag-breakup mechanism of a falling drop; (c) crown splashing of a droplet on a wet surface; and (d) the partial coalescence mechanism of a liquid drop at a liquid–liquid interface. The numerical simulations agree well with available experimental data and theoretical approximations where applicable.

Damped coalescence cascade of liquid drops

During coalescence of a drop with a liquid bath, the damped coalescence cascade (DCC) occurs due to local Marangoni flow generated on the bath surface. The local Marangoni flow accelerates air film drainage between the drop and the bath, leading to suppression of the rebound of daughter drops.

On the stability of the production of bubbles in yield-stress fluid using flow-focusing and T-junction devices

We investigate experimentally the stability of bubble production in yield-stress fluids (YSF) and highly viscous silicone oil, using flow-focusing and T-junction devices. When the exit channel is initially pre-filled with the fluid and the gas is pressure-driven, the production is highly unstable, despite a regular frequency of bubble production in the junction. As observed for pressure-driven bubble trains in Newtonian fluids, we report that two mechanisms can explain these observations: (i) drastic reduction of the hydrodynamic pressure drop along the channel during the transient bubble production, which induces a rapid increase of the gas flow rate and (ii) thin film deposition resulting in a cascade of plug break-up and bubble coalescence. While the drastic reduction of the pressure drop is inevitable in such two-phase flows, we show that modifying the surfaces of the channel can help to stabilize the system when the continuous phase is a YSF. To do so, we measure the thickness of the film deposited on the channel wall for rough and smooth channels. Our results are rationalized by introducing the inverse of the Bingham number Bi−1 comparing the viscous stress to the yield stress. For Bi−1 ≥ 1, a fast fluidization process associated to efficient deposition of YSF on the channel wall leads to a rapid destabilization of bubble production. However, for Bi−1 < 1, the deposition driven by capillarity can be hindered by the wall-slip induced by the existence of the yield stress: the thickness of the deposited film is very thin and corresponds to the equivalent roughness of the channels. It is typically 40 μm thick for rough surfaces and below the limit of resolution of our set-up for smooth surfaces. In this regime of Bi−1 and for smooth surfaces, the length of the plugs barely vanishes, thus the start-up flow is less prone to destabilization. These results therefore potentially open routes to steady production of aerated YSF on smooth channels in the regime of small Bi−1.

Partial coalescence of soap bubbles

We present the results of an experimental investigation of the merger of a soap bubble with a planar soap film. When gently deposited onto a horizontal film, a bubble may interact with the underlying film in such a way as to decrease in size, leaving behind a smaller daughter bubble with approximately half the radius of its progenitor. The process repeats up to three times, with each partial coalescence event occurring over a time scale comparable to the inertial-capillary time. Our results are compared to the recent numerical simulations of Martin and Blanchette [“Simulations of surfactant effects on the dynamics of coalescing drops and bubbles,” Phys. Fluids 27, 012103 (2015)] and to the coalescence cascade of droplets on a fluid bath.

Simulations of surfactant effects on the dynamics of coalescing drops and bubbles

We present simulations of coalescence in the presence of surfactant. We consider a fluid-fluid interface where we track surfactant concentration. Our model is applicable to a soap bubble merging with a suspended soap film and to a surfactant covered liquid drop merging with a reservoir. In both cases, we determine the regime in which coalescence is only partial. Along with viscous effects, represented by the Ohnesorge number, the elasticity of the surface tension relative to the surfactant concentration is seen to play a key role and exhibits a surprising nonmonotonic influence, for which we present a physical mechanism. The effects of gravity are also simulated, along with effects of differing initial conditions, as well as those of uneven initial surfactant concentration, as are likely to arise in physical applications. We describe how the presence of surfactants can influence a coalescence cascade.

Bright minds shine in Los Angeles: The 2014 ISEF

The theme for the 2014 International Science and Engineering Fair (ISEF) was “Bright Minds Shine.” The 2014 SEG award and honorable-mention winners tackled problems related to transforming data to sound, identifying species using Bayes’ theorem, attenuating tsunami waves, extracting signal from noisy data, placing wind turbines in urban areas, and measuring water waves caused by partial coalescence cascades.

Satellite formation during bubble transition through an interface between immiscible liquids

AbstractWhen a bubble rises to an interface between two immiscible liquids, it can pass through the interface, if this is energetically favourable, i.e. the bubble preferring the side of the interface with the lower air–liquid surface tension. Once the intermediate film between the bubble and the interface has drained sufficiently, the bubble makes contact with the interface, forming a triple-line and producing strong capillary waves which travel around the bubble and can pinch off a satellite on the opposite side, akin to the dynamics in the coalescence cascade. We identify the critical Ohnesorge numbers where such satellites are produced and characterize their sizes. The total transition time scales with the bubble size and differential surface tension, while the satellite pinch-off time scales with the capillary-inertial time of the pool liquid, which originally surrounds the bubble. We also use high-speed video imaging to study the motion of the neck of the contact. For low viscosity we show that it grows in time with a power-law exponent between 0.44 and 0.50, with a prefactor modified by the net sum of the three interfacial tensions. Increasing the viscosity of the receiving liquid drop drastically slows down the motion of the triple-line, when the Ohnesorge number exceeds ${\sim }$0.08. This differs qualitatively from the coalescence of two miscible drops of different viscosities, where the lower viscosity sets the coalescence speed. We thereby propose a strong resistance from the triple-line.

Microscopic insight into the nanocoalescence of a water droplet on a water bath

Coalescence of a millimetric water droplet on a water bath has been extensively investigated by experiments, theory and continuum approaches. While the hydrodynamic processes have been largely studied the underlying microscopic mechanisms are much less understood. Thanks to the recent advances in mesoscopic modelling the physics occurring at the nanometric scale can be captured. By using the coarse- and fine-grained simulations we investigate the mechanism of the coalescence of a water nanodroplet on a water bath. In contrast to the millimetric drop we show the absence of a coalescence cascade. The two most probable mechanisms of coalescence largely discussed in the literature are i) the drainage of an air cushion between the water droplet and the bath and ii) the rupture of both water drop and reservoir interfacial layers. From the time-evolution of the 3D water density profile we show that the mechanism begins by the formation of a coalescence bridge during the first picosecond, followed by a temporary noncoalescence where the shape of the drop is modified. The life time of the temporary noncoalescence is ruled by the dissipation of the interfacial layers of the droplet and of the water bath. During this phase the droplet expels water downward through the bridge leading to a thinning of the interfacial layers and to their dissipation that corresponds to the beginning of the drop coalescence into the water bath. In this work we show that the microscopic mechanism of nanocoalescence is partially in line with the millimetric process. Indeed, in contrast with the macroscopic droplet a deformation of the interface was evidenced at the nanometric scale while at the millimetric scale the interfaces suddenly coalesce after the interfacial layers sufficiently dissipated and the drop merges into the bulk.

Propagation of capillary waves and ejection of small droplets in rapid droplet spreading

AbstractA new regime of droplet ejection following the slow deposition of drops onto a near-complete wetting solid substrate is identified in experiments and direct numerical simulations; a coalescence cascade subsequent to pinch-off is also observed for the first time. Results of numerical simulations indicate that the propagation of capillary waves that lead to pinch-off is closely related to the self-similar behaviour observed in the inviscid recoil of droplets, and that motions of the crests and troughs of capillary waves along the interface do not depend on the wettability and surface tension (or Ohnesorge number). The simulations also show that a self-similar theory for universal pinch-off can be used for the time evolution of the pinching neck. However, although good agreement is also found with the double-cone shape of the pinching neck for droplet ejection in drop deposition on a pool of the same liquid, substantial deviations are observed in such a comparison for droplet ejection in rapid drop spreading (including the newly identified regime). This deviation is shown to result from interference by the solid substrate, a rapid downwards acceleration of the top of the drop surface and the rapid spreading process. The experiments also confirm non-monotonic spreading behaviour observed previously only in numerical simulations, and suggest substantial inertial effects on the relation between an apparent contact angle and the dimensionless contact-line speed.