Production and Reconfiguration of Double Emulsions by Temperature Control

Abstract

Double emulsions are of great importance for both science and engineering. However, the production of multicore double-emulsion droplets is challenging and normally requires sophisticated microfluidic devices, which limits their availability to broader communities. Here, we propose a simple, precise, and scalable batch method for producing double emulsions with monodispersed multicores at milliliter per minute rates, using the most common means in laboratory, temperature. By rapidly cooling liquid crystal emulsions, the introduced temperature gradient around the emulsion droplets leads to the injection of monodispersed guest droplets to form double-emulsion droplets. The number of injected water droplets can be precisely controlled by adjusting the thermally induced mechanical force through the temperature difference and the cooling rate. In contrast to conventional microfluidic fabrication, this method processes all emulsion droplets simultaneously in a noncontact and in situ manner. Therefore, it has great flexibility, allows multiple processing of double emulsions of arbitrary shape, has good capacity for mass production, and offers excellent compatibility with technologies such as microfluidics. Finally, we demonstrate that temperature changes can also be used to release the inner droplets from the double emulsion. The proposed method offers a reversible tool for processing double emulsions with minimal cost and expertise and is applicable to droplet-based microsystems in materials science, photonics, sensors, pharmaceuticals, and biotechnology.

Recent Progress in Droplet Structure Machining for Advanced Optics

“…Laser injection technology is an emerging technique for customizing the internal architecture of a droplet in a bottom–up manner [ 28 , 52 , 53 , 54 , 55 ]. The microfabrication is initiated by the injection of nanoscale water droplets into a pre-selected site on the surface of a stabilized LC (one type of soft matter) droplet through laser irradiation; no syringe or nozzle is required ( Figure 6 a).…”

“…Fortunately, the laser injection technology provides an available tool to overcome the surface confinement and loading water droplet cores into the LC droplet through a pre-selected site. Therefore, it achieves complete intervention in the self-assembly dynamics of the injected droplets by means of injecting them into a host LC droplet with a stable orientational field [ 28 , 55 , 61 ]. This eliminates the barrier to machining the internal architecture of a droplet in a bottom–up manner based on self-assembly, resulting in high machining accuracy.…”

“…Specifically, a Gaussian laser beam functions as a localized heat source, inducing instabilities at the water-LC interface and resulting in the injection of water across the interface [ 28 ]. A similar injection process can also be achieved through direct temperature control [ 55 ]. Although the concrete mechanism requires further quantitative studies, it has been confirmed that the interface injection is primarily driven by the thermally induced Marangoni effect.…”

“…Although the concrete mechanism requires further quantitative studies, it has been confirmed that the interface injection is primarily driven by the thermally induced Marangoni effect. As the injection site on the interface is determined by the temperature distribution, and even the total volume of the injected water solution can be precisely controlled by adjusting the induced Marangoni force ( Fσ , Equation (2)) through the applied temperature difference and cooling rate ( Figure 7 ) [ 55 , 62 , 63 ]. where R is the radius of the LC droplets, dσ / dT is the thermal dependence of the surface tension at the LC–water interface, and dT / dr is the temperature gradient over the LC–water interface.…”

“…(a) The induced temperature gradient based on the FEM simulation. (b) Injection site covers the bottom surface of the droplet.Adapted from Ref [55]. with permission from American Chemical Society.…”