Optical Flow Focusing: Light-Induced Destabilization of Stable Liquid Threads

Confinement of flowing liquid threads by solid walls makes them stable with respect to the Rayleigh-Plateau instability. We demonstrate here that light can break this stability by forcing locally the deformation of the liquid interface through thermally induced Marangoni stresses. Depending upon the confining conditions and fluid properties, this optocapillary deformation either pinches or inflates the thread, which may, in both cases, lead to its localized fragmentation into droplets. In the pinching regime, the laser beam behaves as a wall-free constriction that flow focuses the thread, leading to successive regimes of single and multiple periodicity. Light-driven local Marangoni stresses may prove an elegant contactless alternative to control reversibly the thread-to-droplet transition for digital microfluidics.