Shape-tunable wax microparticle synthesis via microfluidics and droplet impact.

Spherical and non-spherical wax microparticles are generated by employing a facile two-step droplet microfluidic process which consists of the formation of molten wax microdroplets in a flow-focusing microchannel and their subsequent off-chip crystallization and deformation via microdroplet impingement on an immiscible liquid interface. Key parameters on the formation of molten wax microdroplets in a microfluidic channel are the viscosity of the molten wax and the interfacial tension between the dispersed and continuous fluids. A cursory phase diagram of wax morphology transition is depicted depending on the Capillary number and the Stefan number during the impact process. A combination of numerical simulation and analytical modeling is carried out to understand the physics underlying the deformation and crystallization process of the molten wax. The deformation of wax microdroplets is dominated by the viscous and thermal effects rather than the gravitational and buoyancy effects. Non-isothermal crystallization kinetics of the wax illustrates the time dependent thermal effects on the droplet deformation and crystallization. The work presented here will benefit those interested in the design and production criteria of soft non-spherical particles (i.e., alginate gels, wax, and polymer particles) with the aid of time and temperature mediated solidification and off-chip crosslinking.

Getting in shape: molten wax drop deformation and solidification at an immiscible liquid interface.

The controlled production of non-spherical shaped particles is important for many applications such as food processing, consumer goods, adsorbents, drug delivery, and optical sensing. In this paper, we investigated the deformation and simultaneous solidification of millimeter size molten wax drops as they impacted an immiscible liquid interface of higher density. By varying initial temperature and viscoelasticity of the molten drop, drop size, impact velocity, viscosity and temperature of the bath fluid, and the interfacial tension between the molten wax and bath fluid, spherical molten wax drops impinged on a cooling water bath and were arrested into non-spherical solidified particles in the form of ellipsoid, mushroom, disc, and flake-like shapes. We constructed cursory phase diagrams for the various particle shapes generated over a range of Weber, Capillary, Reynolds, and Stefan numbers, governed by the interfacial, inertial, viscous, and thermal effects. We solved a simplified heat transfer problem to estimate the time required to initiate the solidification at the interface of a spherical molten wax droplet and cooling aqueous bath after impact. By correlating this time with the molten wax drop deformation history captured from high speed imaging experiments, we elucidate the delicate balance of interfacial, inertial, viscous, and thermal forces that determine the final morphology of wax particles.