Thermal effects on formation dynamics of double emulsions with a non-Newtonian middle phase

This study presents a numerical investigation of the formation dynamics of thermally induced double emulsions in a microfluidic co-flow device. The middle-phase fluid is non-Newtonian, and its shear-thinning behavior is described by the Carreau model. It is found that shear-thinning shortens the breakup length of the double emulsion, reduces the retraction amplitude of the core droplet, and accelerates its deformation toward a spherical shape during downstream migration. Thermal effects critically govern double-emulsion formation by enhancing interfacial disturbances through the Marangoni effect, shortening the formation time, and reducing the droplet size. Meanwhile, the thermally induced reduction in interfacial tension increases the breakup length and improves the concentricity and uniformity of the core–shell structure. The outer-phase flow rate acts as another key parameter controlling double-emulsion formation. Numerical results further reveal a transition from stable encapsulation to no encapsulation at high outer-phase flow rates: beyond a critical Reynolds number, the middle-phase jet breaks up prematurely and stable core–shell encapsulation can no longer be maintained, with shear-thinning middle fluids losing stability earlier than the Newtonian case. Finally, an empirical predictive model for the formation time is proposed by fitting the numerical results. This study provides valuable guidance for the controllable preparation of double emulsions under coupled rheological and thermal effects. • Reveals how the non-Newtonian middle phase affects double emulsion formation. • Indicates that heating accelerates double-emulsion droplet formation. • Demonstrates that higher outer-phase flow can destabilize encapsulation. • Establishes a model for predicting droplet formation time.