Particles transport driven by a temperature gradient in a solution is known as thermophoresis or Soret effect. The drift velocity of a particle is expressed as , where is a thermophoretic mobility. Therefore, the thermophoretic mobility is a parameter to characterise the nature of thermophoresis, and the systematic measurement of for various combinations of particles and solvents is necessary for its potential application. In the present work, we develop the microfluidic system called microgap Soret cell and show its validity by obtaining for wide experimental conditions. It is shown that the microgap Soret cell can rapidly and directly obtain the thermophoretic mobility of a particle with the diameter of by maintaining the large temperature gradient in the microfluidic system. Moreover, by using the microgap Soret cell, the temperature dependency of is investigated for thermophoresis of polystyrene particles in solutions of sodium hydroxide, polyethylene glycol, and glycerol.

A temperature gradient in a continuous fluid induces the motion of dispersed micro- and nanoparticles even when the fluid is motionless. This phenomenon is known as thermophoresis, and it is expected to be the basis for techniques to control particle motion. In this study, we use the thermophoresis of micro- and nanoparticles in a microchannel filled with an aqueous solution to control the particle motion near the inlet of a sudden contraction, which is a narrower channel connecting two wider channels. Microfluidic devices with sudden contractions are widely used to develop various devices with micro- and nanometer dimensions, such as nanopore sensors. A near-infrared laser is used to create a strong temperature gradient of $O({10}^6)\mathrm{K}{\mathrm{m}}^{-1}$ and induce thermophoresis of micro- and nanoparticles. Because the heating by the laser irradiation is localized near the inlet of the contraction, this configuration is useful for controlling particle translocation into and through the contraction. We characterize our experimental setup by quantifying flow and temperature fields near the contraction channel using particle image velocimetry and laser-induced fluorescence, respectively. Then, we observe the obstruction of the particle translocation into the contraction channel induced by the laser-induced thermophoresis for various parameters such as channel dimensions, flow speeds, particle sizes, and laser powers. Near the inlet of the contraction channel, the counterbalance of thermophoretic force and flow drag leads to the ringlike pattern formation of the particle distribution. Moreover, we carry out some demonstrations using the proposed system to selectively translocate particles and enhance the sensing performance due to increased particle density. Thanks to the noncontact nature of laser-induced thermophoresis, the integration of our method into existing microfluidic devices is feasible and expected to improve technologies for manipulating particles in fluids.

We have succeeded in the selective trapping of single gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave laser.