The drop size in membrane emulsification determined from the balance of capillary and hydrodynamic forces

Monodisperse polyhydroxyalkanoate nanoparticles as self-sticky and bio-resorbable tissue adhesives

Monodisperse nanoparticles of biodegradable polyhydroxyalkanoates (PHAs) polymers, copolymers of 3-hydroxybutyrate (3HB) and 4-hydroxybutyrate (4HB), are synthesized using a membrane-assisted emulsion encapsulation and evaporation process for biomedical resorbable adhesives. The precise control over the diameter of these PHA particles, ranging from 100 nm to 8 μm, is achieved by adjusting the diameter of emulsion or the PHA concentration. Mechanical properties of the particles can be tailored based on the 3HB to 4HB ratio and molecular weight, primarily influenced by the level of crystallinity. These monodisperse PHA particles in solution serve as adhesives for hydrogel systems, specifically those based on poly(N, N-dimethylacrylamide) (PDMA). Semi-crystalline PHA nanoparticles exhibit stronger adhesion energy than their amorphous counterparts. Due to their self-adhesiveness, adhesion energy increases even when those PHA nanoparticles form multilayers between hydrogels. Furthermore, as they degrade and are resorbed into the body, the PHA nanoparticles demonstrate efficacy in in vivo wound closure, underscoring their considerable impact on biomedical applications.

Droplet Formation by Confined Liquid Threads inside Microchannels

A confined liquid thread can form monodisperse droplets near the exit of a microchannel, provided the continuous phase is able to enter the microchannel. A general model that accurately predicts the droplet size including the breakup position inside the microchannel is presented and is verified with experimental observations; breakup occurs as long as the capillary number (Ca) of the liquid thread is below a critical capillary number (Ca_cr); for cylindrical microchannels, it is derived that Ca_cr = 1/16. Below Ca_cr, the formed droplets at the exit of the microchannel have a diameter approximately two times the diameter of the liquid thread; around and above Ca_cr, the liquid thread remains stable and the formed droplets grow infinitely large. The presented controlled droplet generation method is a useful tool for producing monodisperse emulsions and has great potential for the food and pharmaceutical industry.

Passive and active droplet generation with microfluidics: a review

Precise and effective control of droplet generation is critical for applications of droplet microfluidics ranging from materials synthesis to lab-on-a-chip systems. Methods for droplet generation can be either passive or active, where the former generates droplets without external actuation, and the latter makes use of additional energy input in promoting interfacial instabilities for droplet generation. A unified physical understanding of both passive and active droplet generation is beneficial for effectively developing new techniques meeting various demands arising from applications. Our review of passive approaches focuses on the characteristics and mechanisms of breakup modes of droplet generation occurring in microfluidic cross-flow, co-flow, flow-focusing, and step emulsification configurations. The review of active approaches covers the state-of-the-art techniques employing either external forces from electrical, magnetic and centrifugal fields or methods of modifying intrinsic properties of flows or fluids such as velocity, viscosity, interfacial tension, channel wettability, and fluid density, with a focus on their implementations and actuation mechanisms. Also included in this review is the contrast among different approaches of either passive or active nature.

Robust scalable high throughput production of monodisperse drops

Monodisperse drops with diameters between 20 μm and 200 μm can be used to produce particles or capsules for many applications such as for cosmetics, food, and biotechnology. Drops composed of low viscosity fluids can be conveniently made using microfluidic devices. However, the throughput of microfluidic devices is limited and scale-up, achieved by increasing the number of devices run in parallel, can compromise the narrow drop-size distribution. In this paper, we present a microfluidic device, the millipede device, which forms drops through a static instability such that the fluid volume that is pinched off is the same every time a drop forms. As a result, the drops are highly monodisperse because their size is solely determined by the device geometry. This makes the operation of the device very robust. Therefore, the device can be scaled to a large number of nozzles operating simultaneously on the same chip; we demonstrate the operation of more than 500 nozzles on a single chip that produces up to 150 mL h^-1 of highly monodisperse drops.

Surface pressure isotherm for a monolayer of charged colloidal particles at a water/nonpolar-fluid interface: experiment and theoretical model

Monolayers from electrically charged micrometer-sized silica particles, spread on the air/water interface, are investigated. Because of the electrostatic repulsion, the distances between the particles are considerably greater than their diameters, i.e., we are dealing with nondensely packed interfacial layers. The electrostatic repulsion between the particles occurs through the air phase. Surface pressure vs area isotherms were measured by Langmuir trough, and the monolayers' morphology was monitored by microscope. The mean area per particle is determined by Delaunay triangulation and Voronoi diagrams. In terms of mean area, the surface pressure for monolayers from polydisperse and monodisperse particles obeys the same law. The experiments show that Π ∝ L(-3) at large L, where Π is the surface pressure and L is the mean interparticle distance. A theoretical cell model is developed, which predicts not only the aforementioned asymptotic law but also the whole Π(L) dependence. The model presumes a periodic distribution of the surface charge density, which induces a corresponding electric field in the air phase. Then, the Maxwell pressure tensor of the electric field in the air phase is calculated and integrated according to the Bakker's formula to determine the surface pressure. Thus, all collective effects from the electrostatic interparticle interactions are taken into account as well as the effects from the particle finite size. By evaporation of water, the particle monolayers are deposited on a solid substrate placed on the bottom of the trough. The electrostatic interparticle repulsion is strong enough to withstand the attractive lateral capillary immersion forces that are operative during the drying of the monolayer on the substrate. The obtained experimental results and the developed theoretical model can be useful for prediction and control of the properties of nondensely packed interfacial monolayers from charged particles that find applications for producing micropatterned surfaces.

Preparation of uniform-sized multiple emulsions and micro/nano particulates for drug delivery by membrane emulsification

Much attention has in recent years been paid to fine applications of drug delivery systems, such as multiple emulsions, micro/nano solid lipid and polymer particles (spheres or capsules). Precise control of particle size and size distribution is especially important in such fine applications. Membrane emulsification can be used to prepare uniform-sized multiple emulsions and micro/nano particulates for drug delivery. It is a promising technique because of the better control of size and size distribution, the mildness of the process, the low energy consumption, easy operation and simple equipment, and amendable for large scale production. This review describes the state of the art of membrane emulsification in the preparation of monodisperse multiple emulsions and micro/nano particulates for drug delivery in recent years. The principles, influence of process parameters, advantages and disadvantages, and applications in preparing different types of drug delivery systems are reviewed. It can be concluded that the membrane emulsification technique in preparing emulsion/particulate products for drug delivery will further expand in the near future in conjunction with more basic investigations on this technique.