Compound Droplet Impact on a Thin Hydrophobic Cylinder.

The impact of compound droplets on solid surfaces is a ubiquitous phenomenon that pervades both the natural and technological fields. A comprehensive understanding of the dynamics of the droplet impact on solid surfaces is therefore of paramount importance for a broad range of applications. In this study, we investigate the impact of a water-in-oil compound droplet on a thin hydrophobic cylindrical surface, with regard to the Weber number and cylinder dimensions. Owing to the prewetting effect of the oil, the droplet completely engulfs the cylinder during impact. The ensuing breakups of oil and water engender various unique impact outcomes, which are depicted via a phase map. The phase boundaries are described by analyzing the gravitational and drag forces exerted by the cylinder. A threshold value of the Weber number is found beyond which its effect on the azimuthal spreading process becomes less obvious. The distinctive axial spreading processes of oil and water are illustrated through high-speed imaging from both front and side perspectives, revealing that droplet oscillation is critically influenced by the Weber number. Our work elucidates the impact dynamics of compound droplets on curved surfaces, providing pivotal insights into related thermal management, droplet printing, and coating fabrication applications.

Tunable and Robust Nanostructuring for Multifunctional Metal Additively Manufactured Interfaces.

Novel processing phenomena coupled with various alloying materials used in metal additive manufacturing (AM) have opened opportunities for the development of previously unexplored micro-/nanostructures. A rationally devised structure nanofabrication strategy of AM surfaces that can tailor the interface morphology and chemistry has the potential for many applications. Here, through an understanding of grain formation mechanisms during AM, we develop a facile method for tuning micro-/nanostructures of one of the most used AM alloys and rationally optimize the morphology for applications requiring low surface adhesion. We demonstrate that optimized AM structures reduce the adhesion of impaling water droplets and significantly delay icing time. The structure can also be altered and optimized for antiflooding jumping-droplet condensation that exhibits significant enhancement in heat transfer performance in comparison to nanostructures formed on conventional Al alloys. In addition to demonstrating the potential of functionalized AM surfaces, this work also provides guidelines for surface-structuring optimization applicable to other AM metals.

Dynamics of alternating current electric field-assisted non-Newtonian droplet formation with geometry confinement.

Recently, microfluidic techniques have been widely applied for biomaterial droplet manipulations due to their precision and efficiency. Many biosamples such as deoxyribonucleic acid and blood samples are non-Newtonian fluids with complex rheology, which brings challenges in control over them. The electric field is characterized by fast response and excellent adaptation to control microscale fluid flow. Here, we systematically investigate the alternating current electric field-assisted formation of non-Newtonian droplet in a flow-focusing microchannel with different sizes of channel orifice. The dependencies of flow conditions, microchannel geometries and electric parameters on the dynamics of non-Newtonian droplet formation are thus elucidated. An effective capacitance electric model is developed to reveal and predict the interaction between the fluid flow and the electric field. Furthermore, the flow field of non-Newtonian droplet formation is captured via the high-speed microparticle image velocimetry system. The characteristics of the regimes of droplet formation and the influences of the channel orifice are revealed quantitatively. Our work offers elaborate references to the control of non-Newtonian droplet formation, which benefits a wide range of applications in biology and chemistry.

Critical conditions for organic thread cutting under electric fields.

Conditions for triggering the cutting of organic samples under an AC electric field are investigated in a microchannel to explore the strategy for organic sample manipulation. Based on the nature of triggering and developing instability at liquid interfaces, in combination with an equivalent electric circuit model, a novel electric capillary number method is proposed as a comprehensive critical condition for the cutting. We uncover the physics behind cutting and non-cutting of an organic thread for different electric frequencies, electric properties of fluid, and width of the organic thread. The critical time required and the critical cutting position are studied to offer guidelines for accurate cutting. Higher electric frequency and higher permittivity of the aqueous phase surrounding the organic phase can reduce the voltage required for cutting. In summary, the newly defined electric capillary number is proved to be a comprehensive criterion for determining the cutting phenomena, which is capable of considering the interfacial tension, the electric permittivity and the electric field strength applied. The results offer applicable references for achieving efficient and accurate cutting of organic samples in practical applications.

Precise morphology control and fast merging of a complex multi-emulsion system: the effects of AC electric fields.

We showed that an AC electric field can be effectively used to control the full morphology of a multi-emulsion system (oil/water/oil, O/W/O and water/oil/water, W/O/W); specifically, the size of outer droplets and the number of inner droplets (from 5 to 0) could be controlled. In our system, such control was achieved by adopting non-contact type of electrodes together with double-flow-focusing geometry to apply an AC electric field during the formation of complex droplets. As such, the AC electric field could be used without contamination. In addition to morphology control, we also achieved both one-step and two-step merging of the core droplets in the W/O/W droplet system within 100 milliseconds, which is by far the fastest merging in double emulsion droplets ever reported. To the best of our knowledge, this paper is the first article to report the control of core droplets in an O/W/O system by matching the frequency of the AC electric field with that of the core production rate. In this article, we adopted the electric capillary number CaE to analyze the effectiveness of the AC electric field applied at a high frequency, which offers a guideline for practical applications. Furthermore, the merging phenomena among various droplet systems discovered could add extra dimensions for the manipulation of double emulsions. Our findings reveal new physical insights that bring about a better understanding of the interfacial phenomena and electrohydrodynamics of droplets, which is of great importance for practical applications involving the complex interactions of multiple droplets.

Electrolyte effect in induced charge electroosmosis.

This paper reports an interesting phenomenon that the velocity increases with increasing sodium chloride (NaCl) concentration in induced charge electroosmosis (ICEO) around a conducting cylinder measured by microparticle image velocimetry (µPIV). It is different from the widely reported velocity decay with increasing electrolyte concentration in AC electroosmosis (ACEO) [M. Z. Bazant et al., MicroTAS, 2007, 2875-2878] and induced charge electrophoresis (ICEP) of Janus particles [S. Gangwal et al., Phys. Rev. Lett., 2008, 100, 058302]. In addition, it is found that a reversed vortex flow emerges in deionized water. As the electric field increases or with a slight addition of NaCl, the ICEO vortex flow recovers. Different from the prediction of thin electric double layer (EDL) models, the observed ICEO flow is asymmetric with respect to the cylinder center. The asymmetry presents an electrolyte dependence, and it is intensified as the applied electric field increases. The ICEO flow obtains maximum values at certain electric field frequencies in NaCl and calcium chloride (CaCl2) solutions. But it is surprisingly insensitive to the electric field frequency in sodium dodecyl sulfate (NaDS) solutions. The optimum electric field frequency varies as electrolyte species change. A linear relationship between ICEO velocity and electric field strength squared is observed in all the examined electrolyte solutions, while the slope varies with the changing electrolyte species.

Droplet manipulation in a microfluidic chamber with acoustic radiation pressure and acoustic streaming.

The present paper reports a novel manipulation method for droplets using acoustic radiation pressure and acoustic streaming. In an acoustic field, droplets deform, oscillate and move in a wide range of applied frequencies. The behavior of a droplet depends on the droplet size, acoustic field and interfacial tension between the two phases. The acoustic field is controlled by the voltage and frequency of the piezoelectric actuator. The results demonstrate a method for low-frequency acoustic actuation of droplets in a microfluidic environment.

Tailoring Light-Weight Aggregates for Concrete 3D Printing Applications.

Concrete 3D printing is a sustainable solution for manufacturing efficient designs and creating less waste, and selecting the optimal materials to use can amplify the advantages of this technology. In this study, we explore printing lightweight concrete by replacing normal weight aggregate with lightweight aggregates such as cenospheres, perlite, and foam beads. We adopt a systematic approach to investigate mixtures using different formulation methods such as the specific gravity and packing factor methods to improve the printing and mechanical performances of the mixtures. The rheological results showed significant improvement in the flow characteristics of the different mixtures using both the specific gravity method and the packing factor method to formulate the mixtures. Furthermore, a statistical tool was used to achieve optimal performance of the mixtures in terms of high specific compressive strength, high flow characteristics, and good shape retention capability by maximizing the specific compressive strength ratio, slump flow, and the static yield stress, while minimizing the slump, dynamic yield stress, and plastic viscosity. With the above design objectives, the optimal percentages of the aggregate replacements (cenosphere, perlite, and EPS foam beads) were 42%, 68%, and 44%, respectively. Finally, the optimized results also showed that the mixture with cenosphere aggregate replacement had the highest specific strength.

Ultrascalable Surface Structuring Strategy of Metal Additively Manufactured Materials for Enhanced Condensation.

Metal additive manufacturing (AM) enables unparalleled design freedom for the development of optimized devices in a plethora of applications. The requirement for the use of nonconventional aluminum alloys such as AlSi10Mg has made the rational micro/nanostructuring of metal AM challenging. Here, the techniques are developed and the fundamental mechanisms governing the micro/nanostructuring of AlSi10Mg, the most common metal AM material, are investigated. A surface structuring technique is rationally devised to form previously unexplored two-tier nanoscale architectures that enable remarkably low adhesion, excellent resilience to condensation flooding, and enhanced liquid-vapor phase transition. Using condensation as a demonstration framework, it is shown that the two-tier nanostructures achieve 6× higher heat transfer coefficient when compared to the best filmwise condensation. The study demonstrates that AM-enabled nanostructuring is optimal for confining droplets while reducing adhesion to facilitate droplet detachment. Extensive benchmarking with past reported data shows that the demonstrated heat transfer enhancement has not been achieved previously under high supersaturation conditions using conventional aluminum, further motivating the need for AM nanostructures. Finally, it has been demonstrated that the synergistic combination of wide AM design freedom and optimal AM nanostructuring method can provide an ultracompact condenser having excellent thermal performance and power density.

Recent advancement in induced-charge electrokinetic phenomena and their micro- and nano-fluidic applications.

Induced-charge electrokinetics (ICEK) remains a hot topic due to its promising applications in micro- and nano-fluidics. Over the past decade, researchers have made a great advancement in both fundamental studies and application developments. They captured (I) a flow reversal in induced-charge electroosmosis (ICEO) and attributed it to the phase delay effect of ions, (II) a chaotic ICEO and attributed it to the concentration polarization in the bulk solution, (III) a non-quadratic correlation for ICEO of non-Newtonian fluids and attributed it to the power-law viscosity, (IV) an induced-charge electrophoretic (ICEP) rotation of Janus doublets, etc. Furthermore, various ICEK-based micro- and nano-fluidic devices have been developed, namely, micropumps, particle focusers, trappers, sorters, and nanopore ion diodes. The present article provides a comprehensive review on the recent advancement of ICEK. Firstly, the fundamental studies of ICEK are introduced; then the micro- and nano-fluidic applications based on ICEK are presented; lastly, promising future directions for both fundamental and applications are discussed. This review presents the basic framework of ICEK, and can facilitate the development of ICEK-based applications.

Net fluid flow and non-Newtonian effect in induced-charge electro-osmosis of polyelectrolyte solutions.

This paper reports an interesting net fluid flow in the induced-charge electro-osmosis (ICEO) of poly(sodium 4-styrenesulfonate) (NaPSS) solutions measured through microparticle image velocimetry (µPIV). The net fluid flow is attributed to the significantly unequal cations and poly-anions of NaPSS. Owing to the phase delay effect of ions, different flow patterns appear with the alternating electric field. The inflow velocity and outflow velocity are found to be unequal and their relative magnitude shows a dependence on the electric field strength. The ICEO velocity is positively correlated with the NaPSS concentration. As NaPSS introduces the non-Newtonian effect, the well-known quadratic relationship between ICEO velocity and electric field strength in Newtonian fluids breaks. The ICEO velocity varies differently with the electric field strength as the NaPSS concentration changes. These new findings can contribute to the understanding of ICEO of complex fluids, e.g., biofluids.

Two-fluid electroosmotic flow in microchannels.

This paper presents a mathematical model to describe a two-fluid electroosmotic pumping technique, in which an electrically non-conducting fluid is delivered by the interfacial viscous force of a conducting fluid; the latter is driven by electroosmosis. The electrical potential in the conducting fluid and the analytical solution of the steady two-fluid electroosmotic stratified flow in a rectangular microchannel was presented by assuming a planar interface between the two immiscible fluids. The effects of viscosity ratio, hold-up, concentration, and interfacial zeta potential are analyzed to show the potential feasibility of this technique.

Acoustofluidic control of bubble size in microfluidic flow-focusing configuration.

This paper reports a method to control the bubble size generated in a microfluidic flow-focusing configuration. With an ultrasonic transducer, we induce acoustic streaming using a forward moving, oscillating gas-liquid interface. The induced streaming substantially affects the formation process of gas bubbles. The oscillating interface acts as a pump that increases the gas flow rate significantly and forms a larger bubble. This method is applicable to a wide range of gas pressure from 30 to 90 kPa and flow rate from 380 to 2700 µL h(-1). The bubble size can be tuned repeatedly with the response time on the order of seconds. We believe that this method will enhance the capability of a microfluidic bubble generator to produce a tunable bubble size.

Electrohydrodynamic instability in an annular liquid layer with radial conductivity gradients.

In this paper, the electrohydrodynamic stability in an annular liquid layer with a radial electrical conductivity gradient is investigated. A weak shear flow arises from a constant pressure gradient in the axial direction. In the radial direction, an electric field is applied. The three-dimensional linear instability analysis is implemented to study the influence of the inner radius, electrical conductivity gradient, shear flow, and ionic diffusion on the dynamics of the fluid layer. It is found that the critical unstable mode may either be oscillatory or stationary. The system becomes more unstable as the dimensionless inner radius a increases. When the inner radius a is small, the critical unstable mode is stationary, while it is given by three-dimensional oblique waves when a is large. When the conductivity gradient is small, the critical unstable mode is the three-dimensional oblique wave, while when the conductivity gradient is large, it would switch to the stationary mode rather than the oscillatory mode. The system becomes more unstable when the Reynolds number is slightly increased from zero. Additionally, it is found that the electrical Schmidt number has dual effects. The liquid layer becomes either more unstable or stable as the electric Schmidt number increases.

Temperature-induced droplet coalescence in microchannels.

This paper reports a technique for temperature-induced merging of droplets in a microchannel. The multiphase system consists of water droplet and oil as the dispersed phase and the carrying continuous phase. A resistive heater provides heating in a rectangular merging chamber. The temperature of the chamber is controlled by the voltage applied to the heater. The merging process of two neighboring droplets was investigated with different applied voltage, flow rate ratio between water and oil and total flowrate. Merging is found to be effective at high flow rate ratio, high temperature, and low total flowrate. The presented technique could be used for merging and mixing in droplet-based lab-on-a-chip platforms.

An electrokinetically tunable optofluidic bi-concave lens.

This paper numerically and experimentally investigates and demonstrates the design of an optofluidic in-plane bi-concave lens to perform both light focusing and diverging using the combined effect of pressure driven flow and electro-osmosis. The concave lens is formed in a rectangular chamber with a liquid core-liquid cladding (L(2)) configuration. Under constant flow rates, the performance of the lens can be controlled by an external electric field. The lens consists of a core stream (conducting fluid), cladding streams (non-conducing fluids), and auxiliary cladding streams (conducting fluids). In the focusing mode, the auxiliary cladding stream is introduced to sandwich the biconcave lens to prevent light rays from scattering at the rough chamber wall. In the diverging mode, the auxiliary cladding liquid has a new role as the low refractive-index cladding of the lens. In the experiments, the test devices were fabricated in polydimethylsiloxane (PDMS) using the standard soft lithography technique. Ethanol, cinnamaldehyde, and a mixture of 73.5% ethylene glycol and 26.5% ethanol work as the core stream, cladding streams and auxiliary cladding streams. In the numerical simulation, the electric force acts as a body force. The governing equations are solved by a finite volume method on a Cartesian fixed staggered grid. The evolution of the interface was captured by the level set method. The results show that the focal length in the focusing mode and the divergent angle of the light beam in the diverging mode can be tuned by adjusting the external electric field at fixed flow rates. The numerical results have a reasonable agreement with the experimental results.

Analysis of chaotic mixing in plugs moving in meandering microchannels.

Droplets moving in meandering microchannels can serve as a passive and robust strategy to produce chaotic mixing of species in droplet-based microfluidics. In this paper, a simplified theoretical model is proposed for plug-shaped droplets moving in meandering microchannels at Stokes flow. With this model to provide the velocity field, particle tracking, which requires a large computation time, is performed directly and easily without interpolation. With this convenience, a broad survey of the parameter space is carried out to investigate chaotic mixing in plugs, including the channel curvature, the Peclet number, the viscosity ratio, and the plug length. The results show that in order to achieve rapid mixing in plugs in meandering microchannels, a large curvature, a small Peclet number, a moderate viscosity ratio, and a moderate plug length are preferred.

Thermal mixing of two miscible fluids in a T-shaped microchannel.

In this paper, thermal mixing characteristics of two miscible fluids in a T-shaped microchannel are investigated theoretically, experimentally, and numerically. Thermal mixing processes in a T-shaped microchannel are divided into two zones, consisting of a T-junction and a mixing channel. An analytical two-dimensional model was first built to describe the heat transfer processes in the mixing channel. In the experiments, de-ionized water was employed as the working fluid. Laser induced fluorescence method was used to measure the fluid temperature field in the microchannel. Different combinations of flow rate ratios were studied to investigate the thermal mixing characteristics in the microchannel. At the T-junction, thermal diffusion is found to be dominant in this area due to the striation in the temperature contours. In the mixing channel, heat transfer processes are found to be controlled by thermal diffusion and convection. Measured temperature profiles at the T-junction and mixing channel are compared with analytical model and numerical simulation, respectively.