Thermophoresis, or, the thermal-induced motion of particles is studied for over three decades. When ions/free particles in a solution experience a temperature gradient, they tend to drift, either towards hot or cold directions, which is determined by their thermo-diffusion coefficients. Engineering the right salt that has ions and counter-ions move in different directions within the solution, i.e., having thermo-diffusion coefficients with opposite signs, a thermoelectric field can be introduced within the fluid. This thermoelectric field depends on several parameters such as ion and counter-ion concentrations, solvent characteristics, other constituents in the liquid, average temperature, and so on. Owing to this complexity, the physics behind the motion of particles under the influence of thermal gradient or thermally induced electric field is not completely understood. The theory for ion motion has been extensively studied using molecular dynamics (MD) simulations but is limited to ions and nanoparticles under constant temperature gradients. Also, studying these interactions between thermoelectric fields and microparticles is also of importance for practical applications, which is not feasible for MD simulations owing to the high computational cost.

Herein, we theoretically and experimentally demonstrate the thermoelectric force on colloidal polystyrene microparticles dispersed in a surfactant (CTAC) solution in a spatially varying temperature gradient field. CTAC is chosen as it results in a thermoelectric field that directs positively charged particles towards the heated region. To date, the theories on thermoelectric force and diffusion velocity are based on a uniform temperature gradient approximation. We show that the simple introduction of microparticles in a uniform temperature gradient distorts this uniformity around the particle due to the discontinuity of thermal conductivity at the interface of the particle, which makes the approximation invalid. Subsequently, we obtain full-scale temperature distribution of the particle in the solution with an external heating source and obtain the spatial variation of thermoelectric field around the particle. Employing an electrostatic framework, we then determine the net thermoelectric force on the particle and compare them to existing models which include a mere correction factor to account of thermal conductivity contrast.  

For experimental validation, a spatially varying temperature gradient is introduced by laser heating of substrate which creates a highly localized (\~5 m) thermal gradient (of the order of 10 K/m). The resultant thermal gradient diffuses the ions (from CTAC, which forms micelles) in a way that the thermoelectric field points towards the laser heating hotspot. The particles are thus trapped at the laser hotspot (on the substrate) and exhibit constrained Brownian motion after trapping. The uncertainty in the position of the particle is converted to trapping stiffness – a measure of how strong the particles are attracted to the laser spot. Lesser the Brownian motion, the higher the trapping stiffness, the higher the stability of the trap. The force profiles obtained from the theoretical model follow Hooke’s law around the heating spot and the obtained theoretical trapping stiffness is compared to the experimental results. An excellent match is observed for varying laser power, different sizes of polystyrene particles, and different concentrations of the surfactant.

This theory is further extended to explain the rotation behavior of Janus particles (with sub-surface thermal conductivity contrast) where the thermoelectric field is converted to thermoelectric torque. Also, laser-induced pulling of Silica particles due to heating of the particles themselves is explained by comparing thermoelectric forces to optical forces. The existing literature fails to explain both these phenomena owing to constant temperature gradient assumption, whereas, the proposed model in this work is successful in determining the particle behavior under laser illumination. Since a 3D simulation software is utilized in obtaining the temperature distribution of the particle, this theory can easily be extended to observe the effects of arbitrarily shaped particles under a thermal gradient for low-powered cell-cell interactions in a spatially controlled thermoelectric field.


Mesoscopic Modeling of Laser-Induced Thermoelectricity Interactions With Polystyrene Microparticles

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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