超亲水多孔表面的小液滴发射行为及动力学特性

构建了4类不同的超亲水微/纳多孔结构,通过低速液滴撞击实验研究了多孔介质的结构参数如微/纳尺度特征、孔隙率以及表面粗糙度等对液滴行为和动力学特性的影响。结果表明：多孔表面液滴的早期扩散符合惯性扩散的幂函数规律,并且纳米级结构孔隙率的变化对幂函数关系无显著影响,微米级结构C和α的值随孔隙率增大而降低,粗糙度的提高能够导致C值增大。发现了两种新颖的小液滴发射模式,分别称为第1阶段断裂和第2阶段断裂。纳米级多孔结构发生第1阶段断裂的原因在于较快的惯性扩散速度和较长的惯性时间;第2阶段断裂发生于微米级多孔结构,原因在于渗透的强化导致液滴高度的快速下降以及惯性时间的缩短。     

Dynamic polygonal spreading of a droplet on a lyophilic pillar-arrayed surface

We experimentally investigated the dynamic polygonal spreading of droplets on lyophilic pillar-arrayed substrates. When deposited on lyophilic rough surfaces, droplets adopt dynamic evolutions of projected shapes from initial circles to final bilayer polygons. These dynamic processes are distinguished in two regimes on the varied substrates. The bilayer structure of a droplet, induced by micropillars on the surface, was explained by the interaction between the fringe (liquid in the space among the micropillars) and the bulk (upper liquid). The evolution of polygonal shapes, following the symmetry of the pillar-arrayed surface, was analysed by the competition effects of excess driving energy and resistance which were induced by micropillars with increasing solid surface area fraction. Though the anisotropic droplets spread in different regimes, they obey the same scaling law S ~ t^2/3 (S being the wetted area and t being the spreading time), which is derived from the molecular kinetic theory. These results may expand our knowledge of the liquid dynamics on patterned surfaces and assist surface design in practical applications.     

Maximum Spreading of Urea Water Solution during Drop Impingement

Droplet impingement of urea water solution (UWS) is a common source for liquid film and solid deposits formed in the tailpipe of diesel engines. In order to better understand and predict wetting phenomena on the tailpipe wall, this study focuses on droplet spreading dynamics of urea water solution. Impingement of single droplets is investigated under defined conditions by high‐speed imaging using shadowgraphy technique. The experimental studies are complemented by numerical simulations with a phase‐field method. Computational results are in good agreement with experimental data for the advancing phase of spreading and the maximum and terminal spreading radius, whereas for the receding phase notable differences occur. For the maximum spreading radius, an empirical correlation derived for glycerol‐water‐ethanol mixtures is found to be valid for millimeter‐sized UWS droplets as well. A numerical simulation for a much smaller droplet however indicates that this correlation is not valid for the tiny droplets of UWS sprays in technical applications.     

随机粗糙表面上剪切变稀流体液滴的沉积过程模拟

采用计算流体力学相场方法模拟了单个剪切变稀非牛顿流体液滴在随机粗糙表面的沉积过程,并分析揭示了随机粗糙表面形貌对液滴运动状态及平衡状态的影响。结果表明,在指定的相同操作条件下,即使在光滑表面,剪切变稀流体液滴比牛顿流体液滴铺展更大且回缩至平衡所需时间更少,不存在二次铺展;剪切变稀流体液滴最大铺展直径随均方根粗糙度Rr与Wenzel粗糙度Wr的增加而略有增加。Wr相同时,随着Rr增大,液滴最终铺展系数减小,高度系数增大,平衡接触面积及接触角有所减小。在Rr相同情况下,随着Wr增大,液滴达到平衡所需时间缩短,平衡接触面积线性增大。     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015     

Spreading of emulsions on a solid substrate

The wettability of emulsions is a prominent factor with a broad impact on an extensive variety of industrial applications ranging from the petroleum to the cosmetic industries. Surprisingly, there is no comprehensive study of emulsion spreading to date. In this work, the spreading of water/silicone oil emulsions on glass substrates was investigated. The emulsions were prepared with varying volume fractions of water dispersed in silicone oil, with addition of small amounts of surfactant to stabilize the emulsion structure. The time-dependent variation of dynamic contact angle, base diameter, and the spreading rate of the emulsion droplets were studied. The effect of water/silicone oil weight percentage as well as the droplet size and dispersed phase bubble size were also investigated. The weight percentage of water/silicone oil emulsion and droplet size did not have a significant impact on the spreading dynamics; however, the dispersed phase bubble size affected the spreading dynamics substantially. The coarsening of the dispersed phase bubbles was the key factor in the distinct spreading behavior of emulsions compared to pure liquids.     

Morphology evolution and dynamics of droplet coalescence on superhydrophobic surfaces

In this work, the coalescence of two equal‐sized water droplets on superhydrophobic surfaces (SHSs) is experimentally investigated. The morphologies of droplet coalescence are observed from side‐view and bottom‐view using high‐speed camera system. The related morphology evolution and dynamics of droplet coalescence are explored. The dynamic behaviors of droplet coalescence on SHSs can be decomposed into liquid bridge growth, contact line evolution, and droplet jumping. The liquid bridge radius is proportional to the square root of time, whereas the dimensionless prefactor is decreased from 1.18 to 0.83 due to the transition of interface curvature. The retraction velocity of the contact line shows limited dependence on initial droplet radii as the retraction dynamics considered here are governed by the capillary–inertial effect. The coalesced droplet finally departs the substrate with a dimensionless jumping velocity of around 0.2. A heuristic argument is made to account for the nearly constant dimensionless jumping velocity. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2913–2921, 2018     

Design of a model‐based feedback controller for active sorting and synchronization of droplets in a microfluidic loop

The transport of confined droplets in fluidic networks can lead to complex spatiotemporal dynamics, precluding full control of the position of droplets in the network. Here, we report the design of a model‐based feedback controller that can actively regulate droplet positions in a network. We specifically consider droplet dynamics in a microfluidic loop where a main channel splits into two and recombines. Consistent with previous studies, we find that without active control, the dynamics of droplets in the loop can range from periodic to chaotic behaviors. However, by implementing the model‐based feedback controller, we show that the droplets can be made to sort alternately into the branches of the loop as well as to synchronize the times at which pairs of droplets exit the loop. In particular, our computations demonstrate that the controller is capable of executing remarkable droplet sort‐synchronization tasks in the otherwise chaotic dynamics in the loop. The design of our controller incorporates a hydrodynamic network model, that is, capable of predicting droplet positions and subsequently delivering an actuation to the branches in the loop through elastomeric valves. Efficacy of the controller is discussed in terms of actuation characteristics and constraints imposed by elastomeric valves. The model‐based feedback controller framework presented in this study is likely to promote the development of lab‐on‐chip technologies in which droplet manipulation tasks are executed with active control. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Hydrodynamic Instabilities of Adhering Droplets Due to a Shear Flow in a Rectangular Channel

This paper reports on the hydrodynamic droplet instabilities of different sizes and viscosities due to shear forces in a rectangular channel. Water‐glycerine droplets of different volumes are investigated. A new and nonambiguous definition for the critical velocity of droplet detachment and a new mathematical correlation between the critical velocity v_crit and the fluid properties are presented. The measurements show that v_crit decreases with the droplet volume but at the same time the contour deformation increases. With increasing viscosity of the liquid droplet, i.e., higher glycerine mass fraction, the contour deformation becomes more prominent and an increase in v_crit can be observed. With respect to the fluid properties and droplet volumes, three different motion patterns are detected.     

Spreading and sorption of a droplet on a porous substrate

In this paper, a model for spreading and sorption of a droplet on a thick porous substrate is derived in the frame of lubrication theory. Flow in the saturated part of the porous material is governed by Darcy's law, assuming a discontinuous wetting front separating the saturated from the unsaturated regions. Numerical results are presented for spreading and sorption of droplets in their dependence on the material and process parameters for axisymmetric and two-dimensional configurations.     

Spreading of liquid droplets on proton exchange membrane of a direct alcohol fuel cell

Spreading of liquid droplets over solid surfaces is a fundamental process with a number of applications including electro-chemical reactions on catalyst surface in membrane electrode assembly of proton exchange membrane (PEM) fuel cell and direct alcohol fuel cell. The spreading process of droplet on the PEM porous substrate consists of two phenomena, e.g., spreading of droplet on PEM surface and imbibition of droplet into PEM porous substrate. The shrinkage of the droplet base occurs due to the suction of the liquid from the droplet into the PEM porous substrate. As a result of these two competing processes, the radius of the drop base goes through a maximum with time. The variation of droplet base and front diameter with time on the PEM porous substrate is monitored using microscope fitted with CCD camera and a PC. It is seen that the droplet base diameter goes through a maximum with time, whereas the front diameter increases continuously with time. Further, methanol droplet spreading and wetting front movement was faster than that for ethanol and deionized water. As the PEM porous substrate is wetted and imbibed well by the methanol compared to ethanol, it is expected that the cross over of methanol would be higher than that of ethanol in direct alcohol fuel cell. It should be noted that cross over of alcohol from anode side to cathode side through membrane is detrimental to the fuel cell operation. The experimental data on the variation of droplet base and wetting front diameter with time is predicted by the model available in the literature.     

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron‐sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid‐particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusion‐controlled burning of the high‐volatile solvent, (2) viscous‐shell formation due to decomposition of the low‐volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4553–4566, 2013     

Deformation of an oil droplet on a solid substrate in simple shear flow

Displacement of immiscible fluids is important in sub-surface processes such as enhanced oil recovery, oil sand processing and detergency. In this study, simulation of an oil droplet deformation on a solid substrate in simple shear flow has been carried out using computational fluid dynamics tool (Fluent 6.3) and the shape of the oil droplet is compared with that of the experimental observation. The dynamic behavior of a two-dimensional oil droplet subject to shear flow in a closed channel is considered under the condition of negligible inertial and gravitational forces. The volume of fluid method is used in Fluent to determine the dynamics of free surface of the oil droplet during the fluid flow. The oil droplet deformation increases with the increase in capillary number, Reynolds number and size of the oil droplet. The deformation of an oil droplet attached to channel surface in simple shear flow is studied experimentally in laminar flow through visual observation using microscope (Ziess, SV11 APO) with high speed camera (PCO). Aniline and isoquinoline was used to form oil droplet and distilled water was used as shearing fluid. The deformation of aniline and isoquinoline droplets was recorded using a high speed camera connected to a PC. The recorded image was replayed and the deformation of aniline and isoquinoline droplets was analyzed using Axio Vision software and compared with the results obtained from CFD simulation. The deformation of different sizes of aniline and isoquinoline droplets at different flow rates of shearing fluid and with time are well predicted by the CFD simulation.     

Full‐physics simulations of spray‐particle interaction in a bubbling fluidized bed

Numerical simulations of a gas‐particle‐droplet system were performed using an Euler‐Lagrange approach. Models accounting for (1) the interaction between droplets and particles, (2) evaporation from the droplet spray, as well as (3) evaporation of liquid from the surface of non‐porous particles were considered. The implemented models were verified for a packed bed, as well as other standard flow configurations. The developed models were then applied for the simulation of flow, as well as heat and mass transfer in a fluidized bed with droplet injection. The relative importance of droplet evaporation vs. evaporation from the particle surface was quantified. It was proved that spray evaporation competes with droplet deposition and evaporation from the particle surface. Moreover, we show that adopting a suitable surface coverage model is vital when attempting to make accurate predictions of the particle's liquid content. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2569–2587, 2017     

Reactive inkjet printing of polyethylene glycol and isocyanate based inks to create porous polyurethane structures

Reactive inkjet printing offers a direct way to create polymeric structures in situ on a substrate. Therefore, two component polyurethane formulations can be utilized to be used in multicomponent inkjet printing. In this contribution, the use of polyethylene glycol (M = 200 g mol⁻¹), glycerol ethoxylate (M = 1,000 g mol⁻¹), and water (blowing agent) in combination with aliphatic 1,6-hexamethylene diisocyanate or aromatic methylene diphenyl diisocyanate for reactive inkjet printing is evaluated. The inks are jettable on a Dimatix DMP-3000 inkjet printer using a 10 pL piezo driven drop-on-demand printhead showing stable droplet formation. Solid films on glass are formed using a drop-by-drop printing strategy. Layer-by-Layer strategy gives best results on polycarbonate substrates forming porous polyurethane structures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46977.     

Monodisperse droplet formation through a continuous jet break‐up using glass nozzles operated with piezoelectric pulsation

In this article, a glass nozzle driven by piezoceramics is used to experimentally study the continuous generation of a stream of monodisperse droplets from the breakup of a liquid jet. The approach is demonstrated by examining the breakup dynamics of three distinct liquids. The droplet formation process has been found to be highly controllable and reproducible. The dependence of nozzle performance on liquid properties, flow rate, and disturbance frequency has been investigated. The ratio between the actual disturbance frequency and Reynolds number, ω, is employed at first to represent the operating range that facilitates monodisperse droplet formation. Then, the dimensionless quantity ω*, obtained by multiplying ω by the viscous characteristic time, r/(μ/ρ), has been shown to collapse the data for the lower and higher bounds of the actual disturbance frequency ranges that secure monodispersity of the droplets of all the samples tested. The impacts of liquid flow rate and disturbance frequency on the monodisperse droplet size and spacing between two neighboring droplets have been investigated and discussed. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

A multicontinuum approach for the problem of filtration of oily water systems across thin flat membranes: I. The framework

A multicontinuum model is built to estimate the permeate flux of an oily water system across a thin flat membrane in cross filtration methodology is demonstrated. Several continua are constructed to represent droplet and pore‐size distribution of both the dispersed oil phase and the porous membrane, respectively. The possible permeation of the oil phase has been divided into three criteria. In the first criterion, oil droplets of a given size range may permeate through a given size range of the porous membrane, in the second criterion, oil droplets of another size range may be rejected through another pore size range, and in the third criterion, oil droplets may break apart leaving a tail inside the pore space, which will eventually permeate, and the rest will sweep off due to shear stress. These protocols identify the methodology of the proposed multicontinuum approach, which is introduced in this first part. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4604–4615, 2017     

Visualizing multiphase flow and trapped fluid configurations in a model three‐dimensional porous medium

We report an approach to fully visualize the flow of two immiscible fluids through a model three‐dimensional (3‐D) porous medium at pore‐scale resolution. Using confocal microscopy, we directly image the drainage of the medium by the nonwetting oil and subsequent imbibition by the wetting fluid. During imbibition, the wetting fluid pinches off threads of oil in the narrow crevices of the medium, forming disconnected oil ganglia. Some of these ganglia remain trapped within the medium. By resolving the full 3‐D structure of the trapped ganglia, we show that the typical ganglion size, as well as the total amount of residual oil, decreases as the capillary number Ca increases; this behavior reflects the competition between the viscous pressure in the wetting fluid and the capillary pressure required to force oil through the pores of the medium. This work thus shows how pore‐scale fluid dynamics influence the trapped fluid configurations in multiphase flow through 3‐D porous media. © 2013 American Institute of Chemical Engineers AIChE J, 59:1022‐1029, 2013     

Effect of viscoelasticity on drop dynamics in 5:1:5 contraction/expansion microchannel flow

Recently, we reported how viscoelasticity affects drop dynamics in a microchannel flow using the finite element-front tracking method (FE-FTM). In this work, we investigate drop dynamics for a wider range of parameters: viscosity ratio between droplet and medium (χ), capillary number (Ca), droplet size, and fluid elasticity. The Oldroyd-B model is adopted as the constitutive equation for the viscoelastic fluid. We observe that the drop deformation in a microfluidic channel is dependent on Ca, which is more pronounced for smaller χ values. The present work shows that viscoelasticity plays an important role in drop dynamics with increasing χ values for Newtonian droplet in viscoelastic medium, which can be attributed to high normal stress developed in narrow film thickness between droplet and channel for higher χ values. We also study circulation problem inside droplets, which is important in practice, such as in droplet reactor application. The present work shows that circulation intensity is enhanced with decreasing χ values. We find that the relevance of viscoelastic effects on internal circulation is dependent on χ values, and the circulation intensity is distinctively decreased with increasing elasticity for high χ values for Newtonian droplet in viscoelastic medium. We expect that the present work be helpful not only in controlling droplets but also to improve our physical insight on drop dynamics in microchannel flows.     

Wetting of hydrophobic substrates by nanodroplets of aqueous trisiloxane and alkyl polyethoxylate surfactant solutions

Trisiloxane surfactants at low concentrations promote the complete and rapid wetting of aqueous droplets on very hydrophobic (hydrocarbon) substrates. This behavior has not been demonstrated by any other surfactant which explains why the trisiloxanes are referred to as superspreaders. Despite many experimental and theoretical investigations the mechanism of superspreading is not fully understood. Molecular dynamics simulations using all-atom force fields have been conducted to attempt to elucidate the mechanism of superspreading. Spherical nanodroplets containing approximately 10,000 water molecules in the bulk and 475 surfactant molecules at the liquid-vapor interface were placed in the vicinity of a graphite substrate and allowed to spread freely at room temperature. In the trisiloxane case the droplet was found to spread very little, although randomly removing 175 surfactant molecules lowered the final contact angle from 110^° to 80^°. In contrast, an alkyl polyethoxylate surfactant-laden droplet was found to spread significantly further, with the equilibrium contact angle reaching 55^°. Similar results for the two surfactant systems were found for cylindrical nanodroplets spreading on a self-assembled monolayer (SAM). The reasons for the lack of spreading in the trisiloxane case and the simulation challenges associated with these systems are discussed. In support of our arguments we demonstrate that the surfactant molecules of an initially uniform aqueous trisiloxane solution self-assemble into a bilayer in tens of nanoseconds on a graphite substrate. Lastly, in a final set of simulations, neat trisiloxane droplets at 450 K are found to arrange into a layered structure on a methyl-terminated SAM and to form a sand pile-shape on a hydroxyl-terminated SAM.