Switching between laser-induced thermophoresis and thermal convection of liquid suspension in a microgap with variable dimension

Opto-thermal manipulation with a 3 µm mid-infrared Er:ZBLAN fiber laser

Water has significantly high absorption around 3 µm wavelength region, originated by its fundamental OH vibrational modes. Here, we successfully demonstrate an opto-thermal manipulation of particles utilizing a 3 µm mid-infrared Er:ZBLAN fiber laser (adjustable from 2700 to 2826 nm) that can efficiently elevate the temperature at a laser focus with a low laser power. The 3 µm laser indeed accelerates the formation of the particle assembly by simply irradiating the laser into water. By altering the laser wavelengths, the assembling speed and size, instantaneous particle velocity, particle distribution, trapping stiffness and temperature elevation are evaluated systematically. We propose that the dynamics of particle assembly can be understood through thermo-osmotic slip flows, taking into account the effects of volume heating within the focal cone and point heating at the focus.

Thermal conductivity effect on thermophoresis of charged spheroidal colloids in aqueous media

Thermophoresis of spheroidal colloids in aqueous media under the thermal conductivity effect is analyzed. The thermophoretic velocity and the thermodiffusion coefficient of spheroidal colloids have been formulated for extremely thin electric double layer (EDL) cases. Furthermore, a numerical thermophoretic model is built for arbitrary EDL thickness cases. The parametric studies show that the thermal conductivity mismatch of particle and liquid gives rise to a nonlinear temperature region around the spheroid, with the thickness close to the minor semiaxis. When the EDL region is thin relative to such nonlinear temperature region, the thermal conductivity effect on the thermophoresis of spheroidal colloids is significant, which strongly depends on the ratio of the minor semiaxis to the EDL thickness, the thermal conductivity ratio of particle to liquid, and the particle aspect ratio. Finally, to estimate the thermodiffusion coefficient of spheroidal colloids with arbitrary thermal conductivity, electrolyte concentration, and particle shape, the average dimensionless axial temperature gradient on the spheroidal equator plane in the EDL region is proposed.

Manipulating nanoparticles based on a laser photothermal trap

A method of efficient directional optical manipulation of nanoparticles based on a laser photothermal trap is proposed, and the influence mechanism of external conditions on the photothermal trap is clarified. Through optical manipulation experiments and finite-element simulations, it is determined that the main cause of gold nanoparticle directional motion depends on the drag force. The laser power, boundary temperature, and thermal conductivity of the substrate at the bottom of the solution and liquid level essentially affect the intensity of the laser photothermal trap in the solution and then affect the directional movement and deposition speed of gold particles. The result shows the origin of the laser photothermal trap and the three-dimensional spatial velocity distribution of gold particles. It also clarifies the height boundary of photothermal effect onset, which clarifies the boundary between light force and photothermal effect. In addition, nanoplastics are manipulated successfully based on this theoretical study. In this study, the movement law of gold nanoparticles based on the photothermal effect is deeply analyzed through experiments and simulations, which is of significance to the theoretical study of the optical manipulation of nanoparticles using the photothermal effect.

An opto-thermal approach for rotating a trapped core-shell magnetic microparticle with patchy shell

The ability to trap and rotate magnetic particles has important applications in biophysical research and optical micromachines. However, it is difficult to achieve the spin rotation of magnetic particles with optical tweezers due to the limit in transferring spin angular momentum of light. Here, we propose a method to obtain controlled spin rotation of a magnetic microparticle by the phoretic torque, which is originated from inhomogeneous heating of the microparticle's surface. The microparticle is trapped and rotated nearby the laser focus center. The rotation frequency is several Hertz and can be controlled by adjusting the laser power. Our work provides a method to the study of optical rotation of microscopic magnetic particles, which will push toward both translational and rotational manipulation of the microparticles simultaneously in a single optical trap.