Multimodal breakup of a double emulsion droplet under an electric field

A portable microfluidic device for thermally controlled granular sample manipulation

Effective granular sample manipulation with a portable and visualizable microfluidic device is significant for lots of applications, such as point-of-care testing and cargo delivery. Herein, we report a portable microfluidic device for controlled particle focusing, migration and double-emulsion droplet release via thermal fields. The device mainly contains a microfluidic chip, a microcontroller with a DC voltage control unit, a built-in microscope with a video transmission unit and a smartphone. Five microheaters located at the bottom of the microfluidic chip are used to unevenly heat fluids and then induce thermal buoyancy flow and a thermocapillary effect, and the experiments can be conveniently visualized through a smartphone, which provides convenient sample detection in outdoor environments. To demonstrate the feasibility and multifunctionality of this device, the focusing manipulation of multiple particles is first analyzed by using silica particles and yeast cells as experimental samples. We can directly observe the particle focusing states on the screen of a smartphone, and the particle focusing efficiency can be flexibly tuned by changing the control voltage of the microheater. Then the study focus is transferred to single-particle migration. By changing the voltage combinations applied on four strip microheaters, the single particle can migrate at predetermined trajectory and speed, showing attractiveness for those applications needing sample transportation. Finally, we manipulate the special three-phase flow system of double-emulsion drops in thermal fields. Under the combined effect of the thermocapillary effect and increased instability, the shell of double-emulsion droplets gradually thins and finally breaks, resulting in the release of samples in inner cores. The core release speed can also be flexibly adjusted by changing the control voltage of the microheater. These three experiments successfully demonstrate the effectiveness and multifunctionality of this thermally actuated microfluidic device on granular manipulation. Therefore, this portable microfluidic device will be promising for lots of applications, such as analytical detection, microrobot actuation and cargo release.

Electrically modulated relaxation dynamics of pre-stretched droplets post switched-off uniaxial extensional flow

Droplets are known to elongate in extensional flow and exhibit capillary instabilities following flow cessation. Under several practical scenarios, where the deformed drops are exposed to electrified environments, the interplay between capillary and electric forces can further modulate the capillary-driven instability that may lead to novel drop evolution, which has not yet been explored. In the present study, we probe the transient droplet deformation under combined electrohydrodynamic and extensional flows, with a particular focus on the relaxation dynamics in a post-elongation phase, as the external flow field is withdrawn while the electric field remains on. Based on pre-relaxed droplet morphology and electric field strength, the drops appear to relax faster or slower, leading to a steady-state or a plethora of breakup events. The slightly deformed drops relax into stable prolate or oblate shape depending on the electrophysical properties of the fluid pairs. On the other hand, under large deformation limit, our results reveal that in the post-elongation phase, the electric field may either stabilize the droplet or may enforce its breakup primarily via two modes: mid-pinching and end-pinching. We have shown that the post-relaxation events can be mapped into the relevant parametric phase space as a function of the relative strengths of the various forcing parameters as well as geometric parameters. These results present new avenues of droplet manipulation in industrial and microfluidic applications by utilizing unique connectivity between the relaxation kinematics and imposed electrical forcing, a paradigm that has hitherto remained unaddressed.

From shaping to functionalization of micro-droplets and particles

The structure of microdroplet and microparticle is a critical factor in their functionality, which determines the distribution and sequence of physicochemical reactions. Therefore, the technology of precisely tailoring their shape is requisite for implementing the user demand functions in various applications. This review highlights various methodologies for droplet shaping, classified into passive and active approaches based on whether additional body forces are applied to droplets to manipulate their functions and fabricate them into microparticles. The passive approaches cover batch emulsification, solvent evaporation and diffusion, micromolding, and microfluidic methods. In active approaches, the external forces, such as electrical and magnetic fields or optical lithography, are applied to microdroplets. Special attention is also given to latest technologies using microdroplets and microparticles, especially in the fields of biological, optical, robotic, and environmental applications. Finally, this review aims to address the advantages and disadvantages of the introduced approaches and suggests the direction for further development in this field.

Electro-Hydrodynamics of Emulsion Droplets: Physical Insights to Applications

The field of droplet electrohydrodynamics (EHD) emerged with a seminal work of G.I. Taylor in 1966, who presented the so-called leaky dielectric model (LDM) to predict the droplet shapes undergoing distortions under an electric field. Since then, the droplet EHD has evolved in many ways over the next 55 years with numerous intriguing phenomena reported, such as tip and equatorial streaming, Quincke rotation, double droplet breakup modes, particle assemblies at the emulsion interface, and many more. These phenomena have a potential of vast applications in different areas of science and technology. This paper presents a review of prominent droplet EHD studies pertaining to the essential physical insight of various EHD phenomena. Here, we discuss the dynamics of a single-phase emulsion droplet under weak and strong electric fields. Moreover, the effect of the presence of particles and surfactants at the emulsion interface is covered in detail. Furthermore, the EHD of multi-phase double emulsion droplet is included. We focus on features such as deformation, instabilities, and breakups under varying electrical and physical properties. At the end of the review, we also discuss the potential applications of droplet EHD and various challenges with their future perspectives.

Effect of Deformation on Droplet Contact Charge Electrophoresis

The effect of deformation on the droplet contact charge electrophoresis (CCEP) was investigated for consistent droplet movement control. Through systematic experiments and numerical simulations, it has been found that overcharging by deformation is up to about 130% of the sphere and is mainly driven by the concentration of the electric field near the tip of the droplet rather than an increase in the surface area. Dimensional analysis revealed a consistent droplet CCEP motion with the electric capillary number range of 0.01-0.09. We also found that the dimensionless droplet charge follows a universal curve proportional to the electric capillary number, regardless of the droplet size, and the weak dependence on the droplet size shown in the experimental results is due to hydrodynamic effects, not electrostatic ones. Changes in droplet velocity distribution with droplet size and the electric capillary number were also investigated. Using the perfect conductor theory and Stokes law, we derived an analytical relationship between the droplet center velocity and the electric capillary number and analyzed the experimental results based on this relationship. This study implies that if proper hydrodynamic correction is applied, the droplet CCEP and its deformation effect can be explained by a perfect conductor theory.

Deformation of Emulsion Droplet with Clean and Particle-Covered Interface under an Electric Field

The electrohydrodynamic deformation of an emulsion droplet with a clean and particle-covered interface was explored. Here, the electrohydrodynamic deformation was numerically and experimentally demonstrated under the stimuli of moderate and strong electric fields. The numerical method involves the coupling of the Navier–Stokes equation with the level set equation of interface tracking and the governing equations of so-called leaky dielectric theory. The simulation model developed for a clean interface droplet was then extended to a capsule model for densely particle-covered droplets. The experiments were conducted using various combinations of immiscible oils and particle suspensions while the electric field strength ~10⁵ V/m was generated using a high voltage supply. The experimental images obtained by the camera were post-processed using an in-house image processing code developed on the plat-form of MATLAB software. The results show that particle-free droplets can undergo prolate (deformation in the applied electric field direction) or oblate deformation (deformation that is perpendicular to the direction of the applied electric field) of the droplet interface, whereas the low-conductivity particles can be manipulated at the emulsion interface to form a ‘belt’, ‘helmet’ or ‘cup’ morphologies. A densely particle-covered droplet may not restore to its initial spherical shape due to ‘particle jamming’ at the interface, resulting in the formation of unique droplet shapes. Densely particle-covered droplets behave like droplets covered with a thin particle sheet, a capsule. The deformation of such droplets is explored using a simulation model under a range of electric capillary numbers (i.e., the ratio of the electric stresses to the capillary stresses acting at the droplet interface). The results obtained are then compared with the theory and experimental findings. It was shown that the proposed simulation model can serve as a tool to predict the deformation/distortion of both the particle-free and the densely particle-covered droplets within the small deformation limit. We believe that this study could provide new findings for the fabrication of complex-shaped species and colloidosomes.

Wall Effects on Hydrodynamic Drag and the Corresponding Accuracy of Charge Measurement in Droplet Contact Charge Electrophoresis

Droplet size dependent wall effects on hydrodynamic drag and the corresponding droplet contact charge estimations were experimentally and theoretically investigated. The consistent reduction in the dimensionless droplet contact charges proportional to droplet size was reported and explained by the parallel and approaching wall effects on the drag coefficient. Extrapolation of the size dependent droplet charge data showed that the droplet charge follows the perfect conductor theory when the droplet radius approaches zero. The proposed model was applied to the drag calculation to estimate and compare dimensionless charges before and after consideration of the wall effects. The droplet free fall test concluded that the droplets in the current experimental setup follow Stokes' law. The theoretical velocity profile of the droplet approaching the wall perpendicularly is proposed considering the approaching wall effect on hydrodynamic drag and verified by comparison with the experiment. The droplet size dependent velocity profile shape change was also explained by this approaching wall effect. The shape of the asymmetric velocity profile along the direction of droplet movement was explained by the effect of the image charge through direct numerical calculation of the electric force. The direct calculation of the electric force also showed that the electric correction at the center of the cuvette is negligible; thus, it is sufficient to consider only hydrodynamic correction for accurate charge measurement in this experimental system. The present study will contribute to the accurate measurement of the droplet charges under contact charge electrophoresis. It also provides the basis for precise control of droplet movement in lab-on-a-chip devices.

Breakups of an encapsulated surfactant-laden aqueous droplet under a DC electric field

We investigate the breakups of an encapsulated conducting aqueous droplet under a direct-current electric field via extensive experiments and theoretical analysis. The encapsulating shell phase and the ambient phase consist of leaky dielectric liquids. We change the surface tension by using an aqueous core with different surfactant (Tween 80) concentrations. Moreover, we vary the core size under different electric-field conditions and observe the core dynamics. We present three different breakup modes of the encapsulated droplet. In the first mode, the encapsulated core forms asymmetric Janus shapes after breakup. In the second and third breakup modes, stable and unstable ternary droplets are formed, respectively. We show that the surfactant molecules significantly alter the dynamics of core stretching. According to the theoretical analysis, we identify the critical conditions of instability leading to breakup. We plot the breakup modes in the form of a phase diagram in the electric capillary number (Ca₂₃ = ε₃r_sE_o²/γ₂₃; ratio of interfacial electric to capillary stresses) vs. radius ratio of the core to the shell (β = r_c/r_s) parametric space at different nondimensional surfactant concentrations (C* = C_Tween 80/C_CMC, where C_CMC represents the critical micellar concentration). The study provides essential physical insight into encapsulated emulsions and is useful for their application in various areas of science and technology.

Compound-Droplet-Pairs-Filled Hydrogel Microfiber for Electric-Field-Induced Selective Release

The separate co-encapsulation and selective controlled release of multiple encapsulants in a predetermined sequence has potentially important applications for drug delivery and tissue engineering. However, the selective controlled release of distinct contents upon one triggering event for most existing microcarriers still remains challenging. Here, novel microfluidic fabrication of compound-droplet-pairs-filled hydrogel microfibers (C-Fibers) is presented for two-step selective controlled release under AC electric field. The parallel arranged compound droplets enable the separate co-encapsulation of distinct contents in a single microfiber, and the release sequence is guaranteed by the discrepancy of the shell thickness or core conductivity of the encapsulated droplets. This is demonstrated by using a high-frequency electric field to trigger the first burst release of droplets with higher conductivity or thinner shell, followed by the second release of the other droplets under low-frequency electric field. The reported C-Fibers provide novel multidelivery system for a wide range of applications that require controlled release of multiple ingredients in a prescribed sequence.