Active optofluidics - Setting fluids in motion by light

Optofluidics refers to a branch of microfluidics in which optics and fluids are integrated to create new optical functions within independently controlled fluid flows  , or vice versa, for controlling fluids using optics  . In the first case, a droplet is formed or a liquid column is created to focus or guide light passively; in the other, the light actively creates its own liquid lens or waveguide. Optohydrodynamics corresponds to the latter case, where light sets fluids in motion, either temporarily or permanently. Indeed, electromagnetic waves possess energy and momentum that can be exchanged with, or transferred to, matter. This results in a stress or force whenever the momentum of the photons undergoes a change in magnitude and/or direction. When photons are reflected and/or transmitted at the interface separating two liquids with different refractive indices, the conservation of momentum results in the emergence of optical radiation pressure, which sets the interface in motion and deforms it; radiation pressure is also one of the mechanisms behind the optical trapping of solid microparticles (optical tweezers, see [E 4 043] ). A volumetric flow can also be induced when photons are elastically scattered in the fluid, a mechanism that underlies optical levitation. Finally, photons can be absorbed by one of the liquids and re-emitted at a lower energy, thereby depositing energy that is often converted into heat. This results in temperature gradients that can trigger forced convection in the volume (thermogravitational coupling linked to the variation of density with temperature) or tangential stresses at the interfaces (thermocapillary coupling linked to the variation of interfacial tension with temperature). Thus, beyond the considerable progress made in the optical trapping of solid...