Control over phase separation and nucleation using an optical-tweezing potential

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a black art. Although there are now numerous examples of control using laser-induced nucleation, a physical understanding is absent and preventing progress. We will show that concentration fluctuations in the neighborhood of a liquid-liquid critical point can be harnessed by an optical-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in liquid mixtures using a low-power laser diode confirm the effect. Phase separation and nucleation through an optical-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter.