Coalescence Dynamics Research Articles

Regulating Ag Wettability via Modulating Surface Stoichiometry of ZnO Substrates for Flexible Electronics

AbstractA novel and highly efficient methodology to regulate (enhance or suppress) the Volmer–Weber 3D growth mode of ultra‐thin (<10 nm) Ag layers by modulating the surface stoichiometry of ZnO substrates prior to Ag deposition is presented. Relative to pristine ZnO layers, oxygen‐deficient surface states formed by preferential removal of surface oxygen atoms remarkably improve Ag layer wettability, whereas oxygen‐excessive surface states formed by oxygen atom incorporation strongly facilitate Ag agglomeration. The dissimilar nucleation and coalescence dynamics are elucidated via combined molecular dynamics and force‐bias Monte Carlo simulations. The improved wettability results in significantly lower sheet resistance in the ultra‐thin (6–10 nm) Ag layers, for example, 6.03 Ωsq−1 at 8 nm, than the previously reported values from numerous other approaches in the equal thickness range. When this unique methodology is applied to ZnO/Ag/ZnO transparent electrodes, simultaneous improvement in electrical conductivity and visible transparency is realized, with a resultant Haacke figure of merit value of 0.139 Ω−1 that is >50% higher than the best reported value for an identically structured electrode. We select transparent heating devices as a model system to confirm that the superior optoelectronic properties are highly sustainable under simultaneous and severe electrical, mechanical, and thermal stresses.

Effects of dynamic adsorption on bubble formation and coalescence in partitioned-EDGE devices

HypothesisDynamic adsorption effects can play a crucial role in bubble formation and stabilization. We hypothesize that microfluidic tools provide direct insights to these effects, and that the final bubble size depends on the intersection of time scales for bubble formation versus adsorption of proteins. ExperimentsWe use a microfluidic device to study Laplace pressure-driven formation of bubbles that are stabilized by whey proteins. Bubble behavior is studied as a function of the pressure difference imposed across the pores (Pd∗), and thus the bubble formation time (τ, ranging from μs to s), using highspeed recordings, quasi-static pressure arguments and a semi-empirical coalescence model. FindingsWe observe two distinct bubble formation regimes, delimited by the pressure difference required to initiate bubble formation in pure water, Pd∗= 1400 mbar. When Pd∗<1400 mbar, protein adsorption is a requisite to lower the surface tension and initialize bubble formation. Individual bubbles (fixed d0~ 25 μm) are formed slowly with τ≫1 ms. When Pd∗ exceeds 1400 mbar, bubbles (fixed d0~ 16 μm) experience no adsorption lag and thus are formed at steeply increasing frequency, with τ < 1 ms. Interaction between these bubbles causes finite coalescence to a diameter dcoal that increases for lower τ. A minimum time of 0.4 ms is needed to immediately stabilize individual bubbles. Our study provides a promising microfluidic tool to study bubble formation and coalescence dynamics simultaneously.

Bouncing and coalescence dynamics during the impact of a falling drop with a sessile drop on different solid surfaces

The dynamic of an impacting drop with a sessile drop of the same liquid on different solid surfaces is investigated experimentally and theoretically. By controlling the surface wettability and the impact velocity, different regimes were observed, such as complete rebounding, direct coalescence, and coalescence during the retraction phase or the spreading phase. It is observed that the complete rebounding phenomenon has widely occurred during the impact on hydrophilic surfaces. In addition, both the maximum and minimum limits of coalescence/complete rebounding thresholds were determined. During the complete rebounding phenomena, and based on the energy balance, the maximum spreading diameter of the falling drop diameter on the sessile drop was proposed. In addition, the restitution coefficient and the contact time between the falling and sessile drops were studied experimentally, and theoretically, the models show a good agreement with the experimental work.

Coalescence dynamics in oil-in-water emulsions at elevated temperatures

Emulsion stability in a flow field is an extremely important issue relevant for many daily-life applications such as separation processes, food manufacturing, oil recovery etc. Microfluidic studies can provide micro-scale insight of the emulsion behavior but have primarily focussed on droplet breakup rather than on droplet coalescence. The crucial impact of certain conditions such as increased pressure or elevated temperature frequently used in industrial processes is completely overlooked in such micro-scale studies. In this work, we investigate droplet coalescence in flowing oil-in-water emulsions subjected to higher than room temperatures namely between 20 to 70 ^{circ }C. We use a specifically designed lab-on-a-chip application for this purpose. Coalescence frequency is observed to increase with increasing temperature. We associate with this observation the change in viscosity at higher temperatures triggering a stronger perturbation in the thin aqueous film separating the droplets. Using the scaling law for rupture time of such a thin film, we establish a mechanism leading to a higher coalescence frequency at elevated temperatures.

Dynamics of two coaxially rising gas bubbles

In this study, the coalescence dynamics of two unequal sized vertically inline bubbles rising in a liquid column have been investigated using the coupled level-set and volume-of-fluid (CLSVOF) method. A wide range of bubble radius ratios of trailing bubble and leading bubble (0.25≤R≤2.0) and separation distances between the bubbles (2.5≤S≤3.5) have been deployed to investigate the evolution of the bubble wakes and bubble shapes. It is discovered that the coalescence time increases with R, the maxima being around 0.75≤R≤1, and then it decreases. With the increase in S, the coalescence time gradually increases. The existence of a pair of counter-rotating vortex rings has been observed between the bubbles, which are seen to accelerate the bubble coalescence process. For the present range of R and S, we show a regime map with four distinct coalescence pathways: coalescence with liquid entrapment, coalescence without liquid entrapment, penetration of the leading bubble, and premature splitting of the trailing bubble.

The Role of Electric Pressure/Stress Suppressing Pinhole Defect on Coalescence Dynamics of Electrified Droplet

The dimple occurs by sudden pressure inversion at the droplet’s bottom interface when a droplet collides with the same liquid-phase or different solid-phase. The air film entrapped inside the dimple is a critical factor affecting the sequential dynamics after coalescence and causing defects like the pinhole. Meanwhile, in the coalescence dynamics of an electrified droplet, the droplet’s bottom interfaces change to a conical shape, and droplet contact the substrate directly without dimple formation. In this work, the mechanism for the dimple’s suppression (interfacial change to conical shape) was studied investigating the effect of electric pressure. The electric stress acting on a droplet interface shows the nonlinear electric pressure adding to the uniform droplet pressure. This electric stress locally deforms the droplet’s bottom interface to a conical shape and consequentially enables it to overcome the air pressure beneath the droplet. The electric pressure, calculated from numerical tracking for interface and electrostatic simulation, was at least 108 times bigger than the air pressure at the center of the coalescence. This work helps toward understanding the effect of electric stress on droplet coalescence and in the optimization of conditions in solution-based techniques like printing and coating.

Modelling of Pore Collapse during Polymer Sintering: Viscoelastic Model with Enclosed Gas.

Frenkel’s model for the late stage of coalescence of viscous particles has been extended to describe pore collapse in a viscoelastic melt during polymer sintering. The shrinkage of a pore in a polymer melt driven by surface tension is extended by taking into account the effects of trapped gas and gas transport out of the pore. Viscoelasticity has been shown to have a considerable impact on the time scale of the coalescence process. In addition, gas diffusion modifies the coalescence dynamics. Based on a parameter study, different regimes for the pore collapse have been identified. At the beginning of pore collapse, surface tension is considerably stronger than gas pressure within the pore. In this time interval (surface-tension-driven regime), the pore shrinks even in the absence of gas diffusion through the matrix. In the absence of gas transport, the shrinkage dynamic slows down and stops when the surface tension balances the gas pressure in the pore. If gas transport out of the pore is possible, surface tension and gas pressure are balanced while the gas pressure slowly decreases (diffusion-controlled regime). The final phase of pore collapse, which occurs when the gas pressure within the pore decreases sufficiently, is controlled again by surface tension. The limitations of the model are discussed. To analyze the interplay between different mechanisms and process steps during selective laser sintering, the respective time scales are compared using experimental data.

Coalescence dynamics of two droplets of different viscosities in T-junction microchannel with a funnel-typed expansion chamber

The coalescence behavior of two droplets with different viscosities in the funnel-typed expansion chamber in T-junction microchannel was investigated experimentally and compared with droplet coalescence of the same viscosity. Four types of coalescence regimes were observed: contact non-coalescence, squeeze non-coalescence, two-droplet coalescence and pinch-off coalescence. For droplet coalescence of different viscosities, the operating range of non-coalescence becomes narrowed compared to the droplet coalescence of same viscosity, and it shrinks with increasing viscosity ratio η of two droplets, indicating that the difference in the viscosity of two droplets is conducive to coalescence, especially when 1＜ η ＜6. Furthermore, the influences of viscosity ratio and droplet size on the film drainage time ( T dr ) and critical capillary number ( Ca c ) were studied systematically. It was found that the film drainage time declined with the increase of average droplet size, which abided by power-law relation with the size difference and viscosity ratio of the two droplets: T dr ~ ( l d ) 0.25±0.04 and T dr ~ ( η ) −0.1±0.02 . For droplet coalescence of same viscosity, the relation of critical capillary number with two-phase viscosity ratio and dimensionless droplet size is Ca c = 0.48 λ 0.26 l −2.64 , while for droplet coalescence of different viscosities, the scaling of critical capillary number with dimensionless average droplet size, dimensionless droplet size difference and viscosity ratio of two droplets is Ca c = 0.11 η −0.07 l s −2.23 l d 0.16 .

Thermally-driven coalescence in thin liquid film flowing down a fibre

Abstract

IMPACT OF LONG-TERM AGEING ON SIGMA PHASE PRECIPITATION PROCESS IN STEELS WITH AUSTENITIC MATRIX

Super 304H, HR3C and Sanicro 25 grade austenitic matrix steels are used in the construction of pressure components of boilers with supercritical operating parameters. The article presents the results of microstructure examination in delivery condition and after age-ing for up to 50,000 hours at 700°C. The microstructure exami-nation was performed using scanning and transmission electron microscopy. The precipitates were identified using transmission electron microscopy. In particular, the study analysed the v phase precipitation process and its dynamics depending on the ageing time. It has been shown that the intermetallic v phase plays a significant role in the loss of durability of the tested steel. It is related to its significant increase due to the influence of high temperature, and its coagulation and coalescence dynamics strongly depend on the ageing/operating temperature level. The qualitative and quan-titative identification of the intermetallic v phase precipitation pro-cess described in the study is important in the analysis of the loss of durability of the tested steels under creep conditions.

Inverse Gaussian distributed method of moments for agglomerate coagulation due to Brownian motion in the entire size regime

The objective of this paper is to derive a model for studying the collision–coalescence dynamics of agglomerates. Accordingly, we first extend the inverse Gaussian distributed method of moments (IGDMOM; J. Atmospheric Sci., 2020, 77(9), 3011–3031) to solve the Smoluchowski coagulation equation for fractal-like agglomerates undergoing Brownian coagulation in the entire size regime. The extended IGDMOM, denoted as E-IGDMOM, is integrated with two solutions, namely Dahneke's solution and the harmonic mean solution, for deriving moment ordinary differential equations (MODEs) for Brownian coagulation in the free molecular regime and in the continuum-slip regime. Comparative analyses demonstrate that the E-IGDMOM achieves nearly the same efficiency as that of the Taylor-series expansion MOM (TEMOM) and the lognormal MOM (log MOM); nevertheless, it has higher precision than that of the log MOM in the free molecular regime and nearly the same precision as that of the TEMOM and log MOM in the transient regime. Regarding the skewness and kurtosis of size distribution functions, compared with the log MOM and IGDMOM, we observe that the E-IGDMOM yields values that are much closer to those of the quadrature MOM.

Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification

Accurate deposition of metal droplets is crucial to limit geometrical defects in metal droplet-based 3D printing. However, unpredictable movements of the droplets displaced from the original landing location due to surface tension-driven flow before it is hindered by solidification, named landing error, lead to broken traces, irregular waved surfaces, or collapsed structures. In the present work, the effect of solidification at the early-time coalescence of two deposited metal droplets on the landing error was revealed. High-speed images were presented to observe coalescence dynamics. A volume of fluid (VOF) based numerical model was developed to analyze the generation mechanism of the landing error. An interesting phenomenon is observed that the second droplet absorbs more mass from the first droplet to form a bigger spherical crown with the increase of substrate temperature, resulting in the distinct final shapes. Inspired by this, a mass conservation model was proposed to predict the dependence of the landing error on substrate temperature. Metal droplets overlapping experiments were further conducted to verify the model and establish the relationship between the landing error and the substrate temperature. Predictions agreed well with experimental results. Finally, a strategy of increasing the thermal conductivity of the substrate to suppress the landing error at high substrate temperatures was proposed, which significantly expanded the processing window of the accurate deposition. This work can provide useful theoretical and experimental guidance for accurate printing of high-quality lines or patterns by utilizing metal droplets.

Experimental and Numerical Study for the Coalescence Dynamics of Vertically Aligned Water Drops in Oil.

In this paper, we propose an experimental and numerical investigation for the impact of the surface tension and the continuous phase viscosity on the dynamics of the liquid bridge during the coalescence process in liquid-liquid systems. A specific configuration of a sessile drop in direct contact with another drop placed over it has been studied. Calculating the redefined Reynolds number Re, it is found that for all studied cases, the coalescence process is dominated by the inertial force. The first step of the work was the validation of the numerical model that has been performed in an axisymmetric coordinate system. This has been done by the comparison between numerical and experimental results obtained in the framework of experimental series realized in parallel for two different liquid-liquid (LL) systems: water drops in silicone oil (SilOil M40.165) and water drops in sunflower oil. A good agreement was found between different results for numerous parameters used for comparisons. It is found that for the first stages of the coalescence (at the start of the drops merging), for a given drop's viscosity, the dynamics of the dimensionless liquid bridge is conducted by the viscosity of the continuous phase where it is illustrated that the more the surrounded viscosity is large, the lower the rate of the liquid bridge growth, the lower the earlier radial velocity of the bridge, and the higher the external capillary pressure generated around the bridge. Moreover, it is depicted that the impact of the surface tension starts appearing after the complete development of the liquid bridge where it is observed that for the same surrounding phase viscosity, the propagation of the capillary wave is faster for a LL system with higher surface tensions than those of lower surface tensions.

Gas bubble dynamics in airlift photo-bioreactors for microalgae cultivation by level set methods

Coupled level set and CFD (computational fluid dynamics) methods are adopted in this work to track the moving gas–liquid interfaces in the riser of an external loop airlift photobioreactor (ALR) in which microalgae are used to produce biofuels and capture CO2 from flue-gas. Modeling the behavior of gas bubbles is a crucial aspect for the fine-tuning of the operation of the reactor when inserted into a closed-loop biorefinery at the pilot-scale. The experimental data used for simulation were completely acquired or calculated from hydrodynamic experimental campaigns carried out on the ALRs. The rise, coalescence, and shape dynamics of the bubbles of the flue-gas are simulated in a rectangular domain representing the vertical section of the ALR riser. Different correction approaches, such as the conservative level set method (CLSM), are proposed to face the volume loss characteristic of LSM. Computational results evidenced strong agreement with the experimental data (bubble shapes and trajectories). The physically-based CLSM model was then effectively used for the fine-tuning of the multiphase flow regime inside the ALRs, suggesting operating conditions for the outdoor cultivation with Reynolds number = 10000 – 11000, Sherwood number between 1400 and 1800, and spherical-caps bubbles in the upper half of the riser, mildly churning the microalgae while avoiding damages to their cells.

Numerical simulations and experiments on droplet coalescence dynamics over a liquid\u2013air interface: mechanism and effect of droplet-size/surface-tension

Present study is on partial/complete coalescence dynamics of a droplet (surrounded by air) over a horizontal pool of the same liquid. Experimental and numerical studies are presented for both isopropanol and glycerol droplet of a constant diameter. Numerical study is presented in more detail for the isopropanol droplet to study the effect of diameter (D=0.035-6.7 mm) and surface tension coefficient (gamma =2-200 mN/m) on the coalescence dynamics. For partial coalescence of an isopropanol droplet and complete coalescence of a glycerol droplet, excellent agreement is demonstrated between our numerically and experimentally obtained interface dynamics; and a qualitative discussion on the mechanism of the partial and complete coalescence is presented. Three regimes of partial coalescence − viscous, inertio-capillary and gravity − proposed in the literature for a liquid-liquid system are presented here for the present liquid-air system while studying the effect of diameter of the isopropanol droplet. Probably for the first time in the literature, our numerical study presents a flow and vorticity dynamics based quantitativeevidence of the coalescence-mechanism, analogy with a freely vibrating Spring-Mass-Damper System, the gravity regime for a liquid-gas system, and the effect of surface tension coefficient gamma based coalescence dynamics study. The associated novel gamma based droplet coalescence regime map presents a critical Ohnesorge number Oh_{c} and critical Bond number Bo_{c} for a transition from partial to full coalescence; and such critical values are also presented for the transition under effect of the droplet diameter. The critical values based transition boundaries, obtained separately for the varying D and varying gamma, are demonstrated to be in excellent agreement with a correlation reported in the literature.

Evaporation-driven nanoparticles motion and deposition on a textured surface in the inkjet process

The evaporation-driven motion and deposition dynamics of nanoparticles (NPs) play significant roles in inkjet-printed structures such as the finger electrodes of solar cells. The objective of the present work is to reveal the mechanisms of NPs motion and distribution on a textured surface in the multi-droplets impact, coalescence and evaporation process. However, the complex interplay among NPs, host fluids and textured medium presents great challenges to numerical modeling. In this paper, a hybrid 3D lattice Boltzmann (LB) model coupled with the Langevin equation (LE) model is presented to handle the multi-droplets impact, coalescence, evaporation and NPs motion dynamics. In the droplet evaporation process, two patterns of NPs deposition rate are first identified at the time of maximum spreading width. For simultaneous and successive impact modes, three peaks of deposited NPs are formed adjacent to the droplet centers after drying out. And the quantities of deposited NPs adjacent to the collision region are increased due to enhanced NPs mixing in successive impacts mode. Detailed figures of NPs motion and deposition on a textured surface are offered that provide in-depth insights beneficial for optimizing the inkjet printing process.

Critical conditions for whether two impacting nanodroplets can coalesce or not: a molecular simulation study.

Molecular dynamics simulations are carried out to study impact-induced coalescence behaviors for the first time. When the droplets possess different impact velocities, the big difference between them could induce unconventional coalescence behaviors that exhibit non-synchronous spreading and retraction processes, and thus produce non-coalescence behaviors. At the same impact velocity, the distance of two impacting droplets plays a vital role in their coalescing dynamics. We here present the lower and upper critical values of distance in a map to determine whether two droplets after impacting can coalesce or not, which are highly dependent on the impact velocity. Moreover, simulation studies show that the upper critical value is 2(Rmax - R), while the lower critical distance increases with the increase of impact velocity. This work not only helps advance our understanding of the effect of impact dynamics on the coalescence behaviors, but also shows the critical conditions for coalescence and non-coalescence behaviors, which could be considered as a new strategy to control the coalescence by tuning the impact parameters, and are expected to be used for some potential applications.

Coalescence speed of two equal-sized nanobubbles

In this work, we use molecular dynamics (MD) simulations coupled with continuum-based theoretical analysis to study the coalescence dynamics of two equal-sized nanobubbles (NBs). We first derive a governing equation for the evolution of the capillary bridge radius between two coalescing NBs from the axisymmetric Navier–Stokes equation. To verify the prediction from the governing equation, we carry out MD simulations of the coalescence of two NBs in a Lennard-Jones fluid system and directly measure the bridge radius, rb, as a function of time, t. By varying the bubble diameter, we change the NB Ohnesorge number from 0.46 to 0.33. In all cases, we find the theoretical prediction overestimates the expansion speed of the capillary bridge at early time of NB coalescence. However, once we take into account the curvature-dependent surface tension and restrict the minimum principal radius at the capillary bridge to the size of the atom in the model liquid, the theoretical prediction agrees with the MD data very well in both early time and later time of the coalescence process. From the theoretical model, we find neither liquid viscous force nor liquid inertial force dominates at later time of coalescence of the model NBs. In this case, the MD simulation results show rb(t) ∝ t0.76 ± 0.04 with the scaling exponent considerably higher than that in the scaling law rb(t) ∝ t0.5 for the viscous and inertial dominated regimes. The diameter ratio of fully merged NB to that of the original NB is about 2, which is different from 23 for the coalescence of millibubbles and microbubbles.

Cloaking Dynamics on Lubricant‐Infused Surfaces

AbstractLubricant‐infused surfaces (SLIPSs/LISs) enable omniphobicity by reducing droplet pinning through creation of an atomically smooth liquid–liquid interface. Although SLIPSs/LISs provide efficient omniphobicity, the need for lubricant adds additional barriers to heat and mass transport and affects three‐phase contact line dynamics. Here, evaporation dynamics of microscale water droplets on SLIPSs/LISs are investigated using steady and transient methods. Although steady results demonstrate that evaporation on SLIPSs/LISs is identical to solid functional surfaces having equivalent apparent contact angle, transient measurements show significant increases in evaporation timescale. To understand the inconsistency, high‐speed optical imaging is used to study the evaporating droplet free interface. Focal plane shift imaging enables the study of cloaking dynamics by tracking satellite microdroplet motion on the cloaked oil layer to characterize critical timescales. By decoupling the effect of substrate material and working fluid via experiments on both microstructured copper oxide and nanostructured boehmite with water and ethanol, it is demonstrated that lubricant cloaking cannot be predicted purely by thermodynamic considerations. Rather, coalescence dynamics, droplet formation, and surface interactions play important roles on establishing cloaking. The outcomes of this work shed light onto the physics of lubricant cloaking, and provide a powerful experimental platform to characterize droplet interfacial phenomena.

Distinct coalescence behaviors of hot and cold drops in the presence of a surrounding viscous liquid

Coalescence of a millimeter-sized drop initially touching a pool of the same liquid in the presence of another surrounding viscous liquid is studied in this work, wherein the drop may be hotter or colder than its surroundings. Moreover, the effect of the outer fluid viscosity on the coalescence dynamics and thermal convection is examined. An axisymmetric numerical model is employed to investigate the drop merger dynamics, wherein the drop and pool are modeled as water fluid, and the surroundings are modeled as silicone oils of different viscosities. The coalescence behaviors of hot and cold drops are found to be significantly different, especially at higher temperature differences. An otherwise partial coalescence for an isothermal system turns into a case of total coalescence when the drop is made colder than its surroundings, whereas the behavior in the case of a hot drop does not depart qualitatively from that of a corresponding isothermal system. Thermal convection has been examined in terms of the penetration depth of hot or cold fluid into the pool. Hot drops are found to have a greater penetration depth as compared to cold drops for higher viscosities of the surrounding fluid. The penetration depth is also related to the size of the leading vortex ring and the maximum vorticity magnitude.