Symmetry breaking in the thermocapillary flow of droplet pair

In cold droplet/hot droplet and cold droplet/hot bath situations, the thermocapillary flow on the liquid surface is axisymmetric and radially along the temperature gradients from high to low regions, inhibiting coalescence by levitating the intervening pressure in the lubricating air film between liquid surfaces. In this work, two droplets at identical temperatures in a relatively cold gaseous phase are studied experimentally. In low-viscosity droplet pairs, symmetry breaking occurs and the thermocapillary flow over each droplet surface is not radially along the temperature gradient from high to low region, but instead manifests as a rotational motion. Moreover, the thermocapillary flows of the two droplets are correlated and always counter-rotating, indicating that the two liquid surfaces are closely correlated. The airflow in the air film is unidirectional, rather than radial. Lubrication theory analysis reveals that non-coalescence is a joint effect of thermocapillary flow and droplet surface deformation, different from the previous situations where only the thermocapillary flow is the deterministic factor. Finally, a criterion for the coalescence/non-coalescence of silicone oil droplet pairs based on the Marangoni number was built. Coalescence occurs when Ma < 230, non-coalescence emerges when Ma > 1000, and the transitional regime lies within 230 < Ma < 1000.