Droplet microfluidics driven by gradients of confinement

Abstract

The miniaturization of droplet manipulation methods has led to drops being proposed as microreactors in many applications of biology and chemistry. In parallel, microfluidic methods have been applied to generate monodisperse emulsions for applications in the pharmaceuticals, cosmetics, and food industries. To date, microfluidic droplet production has been dominated by a few designs that use hydrodynamic forces, resulting from the flowing fluids, to break drops at a junction. Here we present a platform for droplet generation and manipulation that does not depend on the fluid flows. Instead, we use devices that incorporate height variations to subject the immiscible interfaces to gradients of confinement. The resulting curvature imbalance along the interface causes the detachment of monodisperse droplets, without the need for a flow of the external phase. Once detached, the drops are self-propelled due to the gradient of surface energy. We show that the size of the drops is determined by the device geometry; it is insensitive to the physical fluid properties and depends very weakly on the flow rate of the dispersed phase. This allows us to propose a geometric theoretical model that predicts the dependence of droplet size on the geometric parameters, which is in agreement with experimental measurements. The approach presented here can be applied in a wide range of standard applications, while simplifying the device operations. We demonstrate examples for single-droplet operations and high-throughput generation of emulsions, all of which are performed in simple and inexpensive devices.step emulsification | surface tension

Three-dimensional model of surfactant replacement therapy

“…The purpose of this simple tree geometry is to allow us to focus on the fluid mechanical phenomena that govern the eventual distribution and are fundamental to the process. Similar symmetric tree assumptions are common for complicated flows in airways (34,35). Introducing patient-specific lung geometries and generally asymmetric trees are planned for future work, but at this stage would mask asymmetries inherent to the basic fluid mechanics with gravity effects.…”

Droplet Manipulations in Two Phase Flow Microfluidics

“…Building further on this idea, also thin trenches or "rails" were created, effectively creating elongated (directional) energy wells. These allow to steer a droplet in flow along these rails [33], or to fuse two droplets by using two converging rails [34]. Other researchers used similar rails for the fusion and sorting of droplets [35].…”

“…When the neck has become thin enough, it eventually breaks due to Rayleigh-Plateau instability. The step can also be a gradual change in height where the gradient influences the eventual droplet size [34]. The step emulsification method has the advantage that the drop size is mainly controlled by the geometry and the resulting change in Laplace pressure, and less influenced by pressure differences between the liquid phases.…”

Droplet microfluidics: A tool for biology, chemistry and nanotechnology

“…Recently, new methods have been introduced to increase the rate of generating and producing droplets, for example, by introducing gradient confinement [13] or by parallelizing conventional structures [14]. Figure 1 shows key examples of droplet-generating methods.…”