Temperature gradient-induced Marangoni convection has attracted much attention for basic research and applications since it provides an effective means for mass and heat transfer through a liquid surface flow. Here the authors first propose a general principle to enhance such surface flow by hindering its transition to recirculation flow using an external field. They subsequently identify ferrofluid and use it validate the principle since its reduced magnetic susceptibility at higher temperatures will make the heated surface liquid stay on the surface by a thermomagnetic body force. Using a laser beam to create a heated local surface and a magnet beneath the ferrofluid to provide a vertical field, a high speed and long-range Marangoni flow is confirmed experimentally and further supported by computational fluid dynamics simulations. To demonstrate possible applications, the authors show a self-driving pipeless liquid conveyor belt that can efficiently transfer heat from a source to sink without external power. The demonstration of enhanced Marangoni convection opens new avenue to explore interfacial fluid dynamics and its wide applications.

Nonlinear optical property of atomically thin materials suspended in liquid has attracted a lot of attention recently due to the rapid development of liquid exfoliation methods. Here we report laser-induced dynamic orientational alignment and nonlinear-like optical response of the suspensions as a result of their intrinsic anisotropic properties and thermal convection of solvents. Graphene and graphene oxide suspensions are used as examples, and the transition to ordered states from initial optically isotropic suspensions is revealed by birefringence imaging. Computational fluid dynamics is performed to simulate the velocity evolution of convection flow and understand alignment-induced birefringence patterns. The optical transmission of these suspensions exhibits nonlinear-like saturable or reverse saturable absorptions in Z-scan measurements with both nanosecond and continuous-wave lasers. Our findings not only demonstrate a non-contact controlling of macroscopic orientation and collective optical properties of nanomaterial suspensions by laser but also pave the way for further explorations of optical properties and novel device applications of low-dimensional nanomaterials.