(98f) Controlled Production of Double Emulsions in Dual-Coaxial Capillaries Device for Millimeter-Sized Hollow Polymer Spheres

Hollow polymer particles with diameters ranging from nanometers to millimeters have wide applications in various fields such as sensors, drug delivery, food industry, pigments, microcapsules and catalyst loading because of their unique properties. Such particles with different diameters usually lead to different applications. Thin-walled shells with diameters in millimeters have their special applications in the area of high energy density physics such as inertial confinement fusion (ICF) experiments. In the ICF experiment, controlling the size, sphericity and shell thickness uniformity of the targets is particularly important. Therefore, controlling the geometries of the hollow polymer spheres used as mandrels for the fabrication of the ICF targets is quite essential. One important aspect of this task is to control the size and shell thickness of the hollow polymer spheres.

The most commonly used method to produce millimeter-scale polymer shells is the emulsion-based microencapsulation technique which involve three steps: the emulsification of the polymer solution encapsulating other substance(s), the solidification of the middle layers of the double-emulsion droplets, and the removal of the encapsulated substance(s). The first step is quite essential to determine the final sizes and their uniformity of the hollow particles. Therefore, size control of the initial water-oil-water (W/O/W) double emulsions is quite important.

Microfluidic technique based on milli- or micro-channels has its special advantages on the controlled production of double emulsions. Based on the developed milli- or microfluidic devices, numerous studies, experimental or theoretical, have been conducted on the single-emulsion droplet size and double-emulsion formation mechanisms. However, few researches have been done systematically on the sizes of the double-emulsion droplets, especially of the millimeter-scale ones.

In present work, we first designed an easy assembling-disassembling dual-coaxial capillaries device for the production of double-emulsion droplets, where polystyrene dissolved in fluobenzene was employed as the oil phase. Two configurations of the co-flowing devices were employed in the experiments. In the so-called one-step device, the inner capillary tip locates at the same cross-section of the middle capillary tip. Thus, the core drop and the shell drop depart from their capillaries ends simultaneously, forming a double-emulsion drop in one step. While in the so-called two step device, the inner capillary tip locates upstream to the middle capillary tip. The core drop and the shell drop break off from their respective capillaries ends successively, forming a double-emulsion drop in two steps. The effect of the fluids flow rates on the double-emulsion droplets size as well as their formation mechanism was studied comprehensively in the proposed devices. Experimental results implied that slight difference in device configurations would lead to significant difference in droplet formation mechanisms and thus size-control laws. In the two-step device, the inner stream of fluid had little influence on the breakage and the outer diameter of the double-emulsion drops, while in the one-step device the effect was significant. With the produced double-emulsion droplets, millimeter-sized polymer capsules with single hollow cavity and ultra-thin shells were obtained by solidifying the oil layers of the compound drops using solvent evaporation method.