Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids

Facile Microfluidic Fabrication of Biocompatible Hydrogel Microspheres in a Novel Microfluidic Device

Poly(ethylene glycol) diacrylate (PEGDA) microgels with tuneable size and porosity find applications as extracellular matrix mimics for tissue-engineering scaffolds, biosensors, and drug carriers. Monodispersed PEGDA microgels were produced by modular droplet microfluidics using the dispersed phase with 49–99 wt% PEGDA, 1 wt% Darocur 2959, and 0–50 wt% water, while the continuous phase was 3.5 wt% silicone-based surfactant dissolved in silicone oil. Pure PEGDA droplets were fully cured within 60 s at the UV light intensity of 75 mW/cm2. The droplets with higher water content required more time for curing. Due to oxygen inhibition, the polymerisation started in the droplet centre and advanced towards the edge, leading to a temporary solid core/liquid shell morphology, confirmed by tracking the Brownian motion of fluorescent latex nanoparticles within a droplet. A volumetric shrinkage during polymerisation was 1–4% for pure PEGDA droplets and 20–32% for the droplets containing 10–40 wt% water. The particle volume increased by 36–50% after swelling in deionised water. The surface smoothness and sphericity of the particles decreased with increasing water content in the dispersed phase. The porosity of swollen particles was controlled from 29.7% to 41.6% by changing the water content in the dispersed phase from 10 wt% to 40 wt%.

Compact Three-Dimensional Digital Microfluidic Platforms with Programmable Contact Charge Electrophoresis Actuation

Digital microfluidics (DMF) has garnered considerable interest as a straightforward, rapid, and programmable technique for controlling microdroplets in various biological, chemical, and medicinal research disciplines. This study details the construction of compact and low-cost 3D DMF platforms with programmable contact charge electrophoresis (CCEP) actuations by employing electrode arrays composed of a small commercial pin socket and a 3D-printed housing. We demonstrate basic 3D droplet manipulation on the platform, including horizontal and vertical transport via lifting and climbing techniques, and droplet merging. Furthermore, phenolphthalein reaction and precipitation process are evaluated using the proposed 3D DMF manipulations as a proof of concept for chemical reaction-based analysis and synthesis. The threshold voltage (or electrical field) and maximum vertical transport velocity are quantified as a function of applied voltage and electrode distance to determine the CCEP actuation conditions for 3D droplet manipulations. The ease of manufacturing and flexibility of the proposed 3D DMF platform may provide an effective technique for programmable 3D manipulation of droplets in biochemical and medical applications, such as biochemical analysis and medical diagnostics.

Digital Microfluidic Mixing via Reciprocating Motions of Droplets Driven by Contact Charge Electrophoresis

Contact charge electrophoresis (CCEP) is an electrically controllable manipulation technique of conductive droplets and particles by charging and discharging when in contact with the electrode. Given its straightforward operation mechanism, low cost, and ease of system construction, it has gained traction as a versatile and potential strategy for the realistic establishment of lab-on-a-chip (LOC) in various engineering applications. We present a CCEP-based digital microfluidics (DMF) platform with two parallel electrode modules comprising assembled conventional pin header sockets, allowing for efficient mixing through horizontal and vertical shaking via droplet reciprocating motions. The temporal chromic change caused by the chemical reaction between the pH indicator and base solutions within the shaking droplets is quantitatively analyzed under various CCEP actuation conditions to evaluate the mixing performance in shaking droplets by vertical and horizontal reciprocating motions on the DMF platform. Furthermore, mixing flow patterns within shaking droplets are successfully visualized by a high-speed camera system. The suggested techniques can mix samples and reagents rapidly and efficiently in droplet-based microreactors for DMF applications, such as biochemical analysis and medical diagnostics.

Microfluidic High-Throughput Platforms for Discovery of Novel Materials

High-throughput screening is a potent technique to accelerate the discovery and development of new materials. By performing massive synthesis and characterization processes in parallel, it can rapidly discover materials with desired components, structures and functions. Among the various approaches for high-throughput screening, microfluidic platforms have attracted increasing attention. Compared with many current strategies that are generally based on robotic dispensers and automatic microplates, microfluidic platforms can significantly increase the throughput and reduce the consumption of reagents by several orders of magnitude. In this review, we first introduce current advances of the two types of microfluidic high-throughput platforms based on microarrays and microdroplets, respectively. Then the utilization of these platforms for screening different types of materials, including inorganic metals, metal alloys and organic polymers are described in detail. Finally, the challenges and opportunities in this promising field are critically discussed.

Ultraviolet protection of Bacillus thuringiensis through microencapsulation with Pickering emulsion method

An encapsulated formulation of Bacillus thuringiensis (Bt) was produced by the Pickering emulsion technique to improve its activity and stability under UV-A radiation. In this technique latex particles, GO nanosheets, olive oil, ethanol, and water were used to encapsulate Bt in colloidosomes. The protective efficacy of this formulation in protecting Bt subsp. Kurstaki against deactivation by UV-A irradiation was measured, so that spore viability and mortality on Ephestia kuehniella (E. kuehniella) Zeller larvae under UV-A radiation are investigated. According to the results of both tests, encapsulated formulation at a concentration of 0.045% has the highest protection of viability. Hence, colloidosome microcapsule formulations successfully provide good protection against UV radiation.

Controlled diffusion of nanoparticles by viscosity gradient for photonic crystal with dual photonic band gaps

Coalescence of droplets containing nanoparticles has been paid much attention regarding fabrication of functional photonic crystal (PC) patterns. However, most studies focus on the coalescence of droplets containing the same nanoparticles. Currently, an active challenge comes from the coalescence of droplets containing different nanoparticles due to the spontaneous mutual diffusion of different nanoparticles between coalescing miscible droplets driven by the released Gibbs free energy. Such diffusion breaks the self-assembly of nanoparticles into promising PCs with dual photonic band gaps (PBGs). In this work, a viscosity gradient was induced in coalescing droplets containing different nanoparticles to control the diffusion of nanoparticles and impede the diffusion across the coalescing interface. Nanoparticles diffused along the viscosity gradient to droplet surfaces and self-assembled into a period structure which enhanced the interaction of nanoparticles and contributed to impeding the random diffusion between droplets. At the same time, the high viscosity at the coalescing interface slowed down the horizontal movement of nanoparticles further and consequently the diffusion of nanoparticles across the interface was impeded. By use of such controlled diffusion of nanoparticles in the viscosity gradient, PCs with PBGs were achieved. These results demonstrate the controlled diffusion of nanoparticles during the coalescence of miscible droplets to facilely fabricate PCs with PBGs in the absence of an existing external field.

Electrowetting Behavior and Digital Microfluidic Applications of Fluorescent, Polymer-Encapsulated Quantum Dot Nanofluids

Digital microfluidics is a liquid-handling technology capable of rapidly and autonomously controlling multiple discrete droplets across an array of electrodes and has seen continual growth in the fields of chemistry, biology, and optics. This technology is enabled by rapidly switching the wettability of a surface through the application of an electric field: a phenomenon known as electrowetting-on-dielectric. The results reported here elucidate the wetting behavior of fluorescent quantum dot nanofluids by varying the aqueous-solubilizing polymers, changing the size of the nanocrystals, and the addition of surfactants. Nanofluid droplets were demonstrated to have very large changes in contact angle (>100°) by employing alternating current voltage to aqueous droplets within a dodecane medium. The stability of quantum dot nanofluids is also evaluated within a digital microfluidics platform, and the optical properties are not perturbed even under high voltages (250 V). Multiple fluorescent droplets with varying emission can be simultaneously actuated and rapidly mixed (<10 s) to generate a new nanofluid with optical properties different from the parent solutions.

Low voltage driven surface micro-flow by Joule heating

We report a low voltage driven surface microfluidic system simply by Joule heating.

Directed Motion of Metallodielectric Particles by Contact Charge Electrophoresis

We investigate the dynamics of metallodielectric Janus particles moving via contact charge electrophoresis (CCEP) between two parallel electrodes. CCEP uses a constant voltage to repeatedly charge and actuate conductive particles within a dielectric fluid, resulting in rapid oscillatory motion between the electrodes. In addition to particle oscillations, we find that micrometer-scale Janus particles move perpendicular to the field at high speeds (up to 600 μm/s) and over large distances. We characterize particle motions and propose a mechanism based on the rotation-induced translation of the particle following charge transfer at the electrode surface. The propulsion mechanism is supported both by experiments with fluorescent particles that reveal their rotational motions and by simulations of CCEP dynamics that capture the relevant electrostatics and hydrodynamics. We also show that interactions among multiple particles can lead to repulsion, attraction, and/or cooperative motions depending on the position and phase of the respective particle oscillators. Our results demonstrate how particle asymmetries can be used to direct the motions of active colloids powered by CCEP.