In microfluidic chip processing technology, it is usually necessary to generate, transport and manipulate droplets in a microchannel. As the size decreases, the interfacial effect becomes the main factor affecting on fluid flow. Due to the physical mechanism of surface tension changing with temperature, Marangoni convection is generated by the light irradiation, which enables non-contact manipulation of droplets to move and classify them in a microchannel. Such technologies have received widespread attention in the application of microfluidic chips.

Recently, the "Micro/nano-fluid mechanics" research group of the State Key Laboratory of Nonlinear Mechanics of the Institute of Mechanics of the Chinese Academy of Sciences has made progress in study of thermocapillary migration of manipulated droplet by thermal radiation due to the heat absorption on the droplet surface and the influence of interfacial rheology. Thermocapillary migration of a deformed droplet in the combined vertical temperature gradient and thermal radiations with uniform and non-uniform fluxes is first analyzed. The creeping flow solutions show that the deformed droplet has a slender or a cardioid shape, which depends on the form of the radiation flux. The deviation from a sphere depends not only on the viscosity and the conductivity ratios of two-phase fluids, but also on the capillary and the thermal radiation numbers. Moreover, in the roles of interfacial rheology on thermocapillary migration of a deformed droplet, only the surface dilatational viscosity and the surface internal energy can reduce the steady migration velocity, but the surface shear viscosity has not any effects on the steady migration velocity. The surface shear and dilatational viscosities affect the deformation of the droplet by increasing the viscosity ratio of two-phase fluids. The surface internal energy directly reduces the deformation of the droplet. However, the deformed droplet still keeps its original shape without the influence of interfacial rheology. Furthermore, it is found that based on the net force balance condition of the droplet, the normal stress balance at the interface can be used to determine the steady migration velocity, which is not affected by the surface deformation in the creeping flow. From the expressions of the normal/the tangential stress balance, it can be proved that the surface shear viscosity does not affect the steady migration velocity. The results could not only provide a valuable understanding of thermocapillary migration of a deformed droplet with/without the interfacial rheology in a vertical temperature gradient controlled by thermal radiation, but also inspire its potential practical applications in microgravity and microfluidic fields.

This research progress as the title of "Thermocapillary migration of a deformed drop in the combined vertical temperature gradient and thermal radiation" has been published in Physics of Fluids [2023, 35 (3): 034104]. Professor Zuobing Wu from the Institute of Mechanics of the Chinese Academy of Sciences is both the first and corresponding authors. This work has been supported by the National Natural Science Foundation of China and the State Key Laboratory of Nonlinear Mechanics of China.

Paper link: https://doi.org/10.1063/5.0142144

Figure 1. Forms of three radiation fluxes and correspondent deformations of the droplet surface.

Figure 2. Internal and external velocity and temperature fields of steady-state droplet migration under thermal the radiation with three forms.