Laplace pressure versus Marangoni convection in photothermal manipulation of micro droplet

We have numerically demonstrated micro droplet migrations by photothermal manipulation, in order to investigate the driving force and accompanying flows. We focus on an oil (oleic acid) droplet in water that provides a positive temperature dependence of interfacial tension. The present direct numerical simulation employs the volume-of-fluid method and the continuum-surface-force method with consideration of temperature dependency. The driving velocity and force magnitude, which we measured quantitatively, agree with experimental and theoretical data. We have found that the dominant driving force exerted on the microdroplet is the interface normal force, i.e., the Laplace pressure, on a curved interface rather than the tangential force. The tangential component directly triggers the Marangoni convection; however, the induced flows inside the droplet are opposite between the cases of “an oil droplet in water” and “a water droplet in oil”. This may be caused by the large difference between the viscosities of water and oil.