Thermotaxis of a deformable droplet in a confined Poiseuille flow

A droplet under a thermal gradient is known to migrate in a preferential direction, as governed by the variation of its interfacial tension with temperature. Contradicting the outcome of reported asymptotic analysis, here we show that the calculation of the droplet migration by considering the variation of interfacial tension with the imposed thermal field alone may be fundamentally incorrect. This error is attributed to the dynamically evolving interfacial temperature field due to a two-way coupling between the thermal field and the flow field, mediated by the droplet deformation and thermal diffusion. By directly capturing an inherent nonlinear coupling between the thermal field and the flow field using explicit interface tracking in a three-dimensional space, our boundary integral based analysis reveals that a linearly decreasing temperature profile imposed along the direction of a plane Poiseuille flow enhances the migration speed of the droplet in both the axial and cross-stream directions. This is in sharp contrast to a prediction of decelerated motion of the droplet under the same imposed thermal field, as obtained from asymptotic theory. We attribute this discrepancy to an alteration of the surface tension mediated interfacial stress due to the locally evolving temperature field, and a consequent concomitant alteration in the interfacial viscous stress to realize a tangential force balance at the interface. From scaling arguments, we show that the resulting change in the viscous drag force may occur over an order of magnitude, disrupting the outcome as compared to that obtained from asymptotic analysis. These results are likely to bear significant implications in controllable separation and sorting of deformable entities in confined fluidic media.

Automated summary

The migration of a droplet under a thermal gradient is influenced by the variation of its interfacial tension with temperature. However, the traditional asymptotic analysis that considers only the variation of interfacial tension with the imposed thermal field may be fundamentally incorrect due to the coupling between the thermal field and the flow field through droplet deformation and thermal diffusion. Explicit interface tracking in three-dimensional space reveals that a linearly decreasing temperature profile enhances the droplet migration speed contrary to the prediction from asymptotic theory. This discrepancy is attributed to the alteration of the interfacial stress caused by the locally evolving temperature field.