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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.

Clean water for human consumption is, in many places, a scarce resource, and efficient schemes to purify water are in great demand. Here, we describe a method to dramatically increase the efficiency of a photocatalytic water purification microreactor. Our hierarchical optofluidic microreactor combines the advantages of a nanostructured photocatalyst with light harvesting by base substrates, together with a herringbone micromixer for the enhanced transport of reactants. The herringbone micromixer further improves the reaction efficiency of the nanostructured photocatalyst by generating counter-rotating vortices along the flow direction. In addition, the use of metal-based substrates underneath the nanostructured catalyst increases the purification capacity by improving the light-harvesting efficiency. The photocatalyst is grown from TiO 2 as a nanohelix film, which exhibits a large surface-to-volume ratio and a reactive microstructure. We show that the hierarchical structuring with micro- to nanoscale features results in a device with markedly increased photocatalytic activity as compared with a solid unstructured catalyst surface. This is evidenced by the successful degradation of persistent aqueous contaminants, sulfamethoxazole, and polystyrene microplastics. The design can potentially be implemented with solar photocatalysts in flow-through water purification systems.

Fog on the automotive windshield interferes with the sight of the driver and causes serious safety problems; thus, removing it rapidly has been a longstanding goal in the automobile industry. In this study, we propose a novel method for improving the defogging performance of the defroster without its structural alteration by using vortex generators (VGs). To optimize the performance of the VGs, we performed particle image velocimetry experiments, an evaporation measurement experiment, and a numerical simulation for visualizing the airflow from the defroster inlet. The dimensions of the VG (height h  =  δ and length ℓ  = 5 δ ) were selected according to the boundary-layer thickness, δ , of the lowest-flow rate ( u low  = 0.33 m/s) used in the defroster inlet. We employed a dimensionless parameter, i.e., the secondary flow intensity ( Se ), to quantify the intensity of vortices for several angles of attack (30°, 45°, and 60°). The results indicated that the vortices generated from VGs with a 30° angle of attack retained their maximum intensity until they reached the vision area of the driver. Additionally, we explored two different configurations of VGs—co-rotating and counter-rotating—and concluded that the counter-rotating configuration had higher performance than the co-rotating configuration. The optimized VGs were directly inserted in the defroster inlet and improved the defogging performance by approximately 10%. The proposed method is applicable to various automobile models for enhancing the performance of the defroster regardless of the interior volume, air-ventilation performance, and other conditions, such as the number of passengers and outside temperature.

We designed a modular microfluidic system for generating droplets using a three-dimensional (3D) printer. This system was manufactured as a modular system with multipurpose droplet generation flexibility. The various types of emulsion droplets can be generated by changing the combination of the incorporated modules. All modules are interconnected based on a novel coupling system that facilitates assembly and disassembly, and leakages do not occur even for pressures of the order of ~ 40 kPa. We demonstrate the capacity of the system to generate emulsion droplets, ranging from single to complicated dual-core double-emulsion droplets. Droplet sizes in the range of 50–500 μm were obtained by controlling the flow rate and the generation frequency in the range of 1–100 Hz. Furthermore, an electrode module was developed to demonstrate different electrohydrodynamic phenomena, such as the shape deformation, coalescence, breakup, and others. This 3D printed modular microfluidic system makes it possible to meet the needs of the end-user, and can be applied to bioassays, material synthesis, and other applications.

We report an experimental investigation of pressure-driven flow of a viscous liquid across thin polydimethylsiloxane (PDMS) membranes. Our experiments revealed a nonlinear relation between the flow rate $Q$ and the applied pressure drop $\\unicode[STIX]{x0394}p$ , in apparent disagreement with Darcy’s law, which dictates a linear relationship between flow rate, or average velocity, and pressure drop. These observations suggest that the effective permeability of the membrane decreases with pressure due to deformation of the nanochannels in the PDMS polymeric network. We propose a model that incorporates the effects of pressure-induced deformation of the hyperelastic porous membrane at three distinct scales: the membrane surface area, which increases with pressure, the membrane thickness, which decreases with pressure, and the structure of the porous material, which is deformed at the nanoscale. With this model, we are able to rationalize the deviation between Darcy’s law and the data. Our result represents a novel case in which macroscopic deformations can impact the microstructure and transport properties of soft materials.

The self-organization of cytoskeletal biopolymers, such as microtubules (MTs), depends on mechanosensing and adaptation to confined spaces such as cellular protrusions. Understanding how these active biopolymers coordinate their formation under confinement leads to advances in bioengineering. Here we report the self-organization of branched MT networks in channels with narrow junctions and closed ends, mimicking cellular protrusions. We find that branching MT nucleation occurs in the post-narrowing region only if this region exceeds a minimum length, determined by MT dynamic instability at the closed end and the timescale for nucleation at a distant point. We term this feedback ‘boundary sensing’. Increasing the amount of branching factor TPX2 in the system accelerates MT nucleation and adjusts this minimum length, but excess TPX2 stabilizes MTs at the closed end, disrupting network formation. We performed experiments and simulations to study how this tunable feedback, wherein growing MTs navigate confinement and create nucleation sites, shapes MT architecture. Our findings impact the understanding of MT self-organization during axonal growth, dendrite formation, plant development, fungal guidance and the engineering of biomaterials. Uncovering the rules of microtubule network self-organization under confinement is key to understanding how cells build structure in complex environments. This study reveals a tunable boundary-sensing feedback mechanism, wherein pioneer microtubules navigate confined environments and generate nucleation sites for new microtubules, thereby shaping network architecture.

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.

Radio-cephalic arteriovenous fistula (RC-AVF) is an operation performed to achieve vascular access for hemodialysis. Although RC-AVF is a reliable and well-known method, this technique presents high rates of early failure depending on the vessel condition. These failures are due to blood shear stress around the anastomosis site and the vascular access failure caused by thrombosis secondary to stenosis formation, as well as vascular access reocclusion after percutaneous interventions. In this work, we fabricate in vitro 3D RC-AVF by using polydimethylsiloxane and 3D printing technology to understand the underlying mechanism and predict AVF failure. Micro- Particle image velocimetry (μ-PIV) focusing on the cardiac pulse cycle is used to measure the velocity field within the artificial blood vessel. Results are confirmed by numerical simulation. Accordingly, the in vitro AVF model agrees well with the simulations. Overall, this research would provide the future possibility of using the proposed method to reduce in vivo AVF failure for various conditions.

Multiplexed methods for nucleic acid detection are immensely challenging to deploy outside of laboratory settings. Conversely, field-deployable methods are limited to low levels of multiplexing. During the COVID-19 pandemic, we developed Streamlined Highlighting of Infections to Navigate Epidemics (SHINE), a sensitive and deployable CRISPR-based technology for nucleic acid detection. Here, we introduce microfluidic SHINE (mSHINE) which enables >100-plex nucleic acid detection using a highly portable microfluidic manifold. The manifold directs a diluted sample into individual reaction chambers, each of which contains lyophilized SHINE reagents and a microscopic stir bar or bead for mixing. Samples can be loaded using a syringe by hand, greatly simplifying the testing process. A subsequent sealing step allows for >100 SHINE reactions to proceed independently and in parallel. We demonstrate that mSHINE has equal sensitivity to SHINE, allowing for highly multiplexed pathogen detection in ≤ 1 hour. In addition, mSHINE can detect single-nucleotide variants, including mutations associated with drug susceptibility. mSHINE shifts the paradigm of laboratory-based multiplexed nucleic acid testing, greatly benefiting patients and public health.

Multiplexed methods for nucleic acid detection are immensely challenging to deploy outside of laboratory settings. Conversely, field-deployable methods are limited to low levels of multiplexing. Here, we introduce Scalable On-site Nucleic Acid Testing Architecture (SONATA), enabling >100-chamber reaction partitioning for multiplexed nucleic acid amplification and detection with a portable microfluidic manifold. The manifold directs a diluted sample into individual reaction chambers, each of which contains lyophilized reagents and a small stir bar or bead for mixing. Samples can be loaded using a syringe by hand, greatly simplifying the testing process. We demonstrate the integration of the platform with Streamlined Highlighting of Infections to Navigate Epidemics (SHINE), a sensitive and deployable CRISPR-based detection technology. We show that deployed with SONATA, SHINE retains its sensitivity, enabling highly multiplexed pathogen detection in ≤ 1 hour. In addition, we demonstrate the detection of single-nucleotide variants, including mutations associated with drug susceptibility.