Janus membrane emulsification for facile preparation of hollow microspheres

Abstract

Hollow microspheres have aroused considerable interests in the materials community owing to their numerous potential applications; however, scalable, facile, low-cost production of hollow microspheres has not yet been demonstrated. Here, inspired by the idea that a hydrophilic surface allows the rapid detachment of gas bubbles to reduce the bubble size significantly, a novel and feasible approach to preparing gas-in-oil-in-water (G/O/W) emulsions on a large scale using Janus membrane emulsification is presented. A Janus membrane with asymmetric wettability, rather than a fully hydrophilic or a fully hydrophobic membrane, is employed in the emulsification system, in which the hydrophobic side allows gas/oil permeation and the hydrophilic side achieves fast release of oily bubbles. The wetting and chemical properties of each side of the membrane were investigated by contact angle and spectroscopy measurement. It was observed that the stability of the emulsions increased with increasing solution pH values. The variation of droplet size and distribution with the gas flow rate was also studied. The outer diameters of double emulsion drops are in the order of 100–300 μm. These droplets are further exposed to UV light to obtain hollow polymeric microspheres through solidification of the oil layers. Hollow structures of the microspheres were verified by HIM observations. This simple strategy opens a new avenue for generating double emulsions and hollow microspheres at scale with relatively narrow polydispersity.