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.

In this study, we have demonstrated the fabrication of a microbeam array (MBA) with various thicknesses and investigated the suitability it for an acoustic sensor with wide-range frequency selectivity. For this, an MBA composed of 64 beams, with thicknesses varying from 2.99–142 µm, was fabricated by using single gray-scale lithography and a thick negative photoresist. The vibration of the beams in air was measured using a laser Doppler vibrometer; the resonant frequencies of the beams were measured to be from 11.5 to 290 kHz. Lastly, the frequency range of the MBA with non-uniform thickness was 10.9 times that of the MBA with uniform thickness.

We experimentally and theoretically characterize dielectric nano- and microparticle orbital motion induced by an optical vortex of the Laguerre–Gaussian beam and investigate the role of hydrodynamic inter-particle interaction.

We study shear-induced instabilities of lipid bilayers immersed in water using coarse-grained molecular dynamics simulations. The shear imposed by the flow of the water induces initially microscopic structural changes of the membrane, starting with tilting of the molecules in the direction of the shear. The tilting propagates in the spanwise direction when the shear rate exceeds a critical value and the membrane undergoes a bucklinglike deformation in the direction perpendicular to the shear. The bucklinglike undulation continues until a localized Kelvin-Helmholtz-like instability leads to membrane rupture. We study the different modes of membrane undulation using membranes of different geometries and quantify the relative importance of the bucklinglike bending and the Kelvin-Helmholtz-like instability of the membrane.

Thermophoresis is the motion of micro- or nanoparticles driven by the temperature gradient of the surrounding fluid. We develop an on-chip microfluidic device to induce the thermophoresis of microparticles. It is clarified that both the bulk material and the chemical modification to the surface affect the thermophoresis of particles in an aqueous solution, even regarding the direction of particle motion. Polystyrene (PS) beads with a diameter of $1\mu \mathrm{m}$ show a thermophobic response to a temperature gradient induced on the developed microfluidic device and move toward the colder region. The interplay between a uniform flow in the microchannel and thermophoresis leads to a dynamic pattern formation of the PS beads in the upstream region. Thus, a purified solution can be obtained in the downstream region, demonstrating the potential of thermophoresis as a microparticle filter. Furthermore, silica beads with a diameter of $1\mu \mathrm{m}$ move toward the hotter region on the device. Therefore, the PS and silica beads show the opposite response to the temperature gradient despite having the same diameter and surface modification. The thermophoretic separation of a mixture of the PS and silica beads is demonstrated.