Thermoviscous flows for microfluidic manipulation*

Recent microfluidic experiments have explored the precise positioning of micron-sized particles via laser-induced thermoviscous flow. From micro-robotics to biology at the subcellular scale, applications of this versatile technique have been realised in a wide range of disciplines. Through the interplay between thermal expansion and thermal viscosity changes, the repeated scanning of the laser along a scan path results in fluid flow and hence net transport. In microfluidic settings, geometry has a significant influence on the flow induced by the focused light. Achieving high-precision microfluidic manipulation of particles in complex environments therefore requires innovative design of laser scan patterns, along with quantitative theoretical understanding. Here we present an analytical, theoretical model for the flow induced by arbitrary scan patterns in complex geometries, showing excellent agreement with new experiments. Our results will enable refined control over particles at the microscale in complex geometries, as well as new studies probing the role of physical transport in living cells experimentally.