photophoresis

Photophoresis is the motion of small particles in a fluid that is driven by light-induced temperature gradients. The effect is observed in both gases and liquids, where illumination causes a thermal imbalance around or within a particle, generating a force that moves the particle relative to the surrounding medium. In aerosols and other gas-phase systems, photophoretic forces can cause noticeable transport of absorbing or semi-absorbing particles. In liquids, the related phenomenon often describes thermophoresis, where a temperature gradient drives diffusive motion of particles, but light can create the necessary gradients to produce directed movement as well, sometimes in more complex or self-generated ways.

Mechanisms differ by medium. In gases, a particle absorbs light on its illuminated side, creating a temperature gradient across the particle. The surrounding gas molecules collide with the particle asymmetrically, imparting a net force that often pushes the particle away from the hotter side and toward or along the gradient depending on properties such as thermal accommodation and particle surface characteristics. In liquids, a temperature gradient establishes a surface slip velocity due to the temperature dependence of interfacial forces and solvent properties; asymmetrically heated particles, including Janus particles with different coatings, can achieve self-propulsion under illumination.

Applications of photophoretic phenomena include optical manipulation and sorting of microparticles, light-driven self-assembly and propulsion in microfluidic systems, and aerosol manipulation in environmental or industrial contexts. The strength of the effect depends on particle size, absorption, thermal properties, and illumination intensity, and practical use often requires balancing sufficient heating with avoidance of damage to sensitive samples.

Historically, photophoresis was identified in aerosol research and has since been developed into a broader framework for light-induced particle transport in both gases and liquids, with ongoing research into nanoscale propulsion and controlled assembly.