Thermophoretic torque in colloidal particles with mass asymmetry

Abstract

We investigate the response of anisotropic colloids suspended in a fluid under a thermal field. Using nonequilibrium molecular dynamics computer simulations and nonequilibrium thermodynamics theory, we show that an anisotropic mass distribution inside the colloid rectifies the rotational Brownian motion and the colloids experience transient torques that orient the colloid along the direction of the thermal field. This physical effect gives rise to distinctive changes in the dependence of the Soret coefficient with colloid mass, which features a maximum, unlike the monotonic increase of the thermophoretic force with mass observed in homogeneous colloids.

Thermal orientation and thermophoresis of anisotropic colloids: The role of the internal composition

“…In the case of a heavy bead, cos θ < 0 indicates that in this model, the heavy bead points towards the hot region. This is consistent with the results obtained previously for the same model [22]. Simulations of diatomic molecules, and binary mixtures, indicate that the heavy component accumulates in the cold region [15,33] Our result for the thermal orientation of the colloid shows that the response to the thermal gradient cannot be interpreted only in terms of the position of the heavy bead in the colloid.…”

“…It was concluded that these particles undergo anisotropic thermophoresis. Recently, it was shown that the internal composition of the particles, specifically a mass gradient, results in a thermophoretic torque [22]. The existence of this torque has been corroborated in both colloidal suspensions consisting of small and large colloids, relative to the characteristic diameter of the solvent molecules.…”

“…It has been shown in previous work [22] that the Soret coefficient of symmetric rods (i.e. with a homogeneous mass distribution) varies with the colloid mass according to,…”

“…where B and C are constants that depend linearly on the size of the colloid (see ref. [22]). It has been assumed in eq.…”

“…The existence of this torque has been corroborated in both colloidal suspensions consisting of small and large colloids, relative to the characteristic diameter of the solvent molecules. For the small colloid/solvent size ratio, the existence of thermophoretic torques was observed using non-equilibrium molecular dynamics simulations [22], while for the large colloid/solvent size ratio the simulations were performed using a non-equilibrium Stochastic Rotation Dynamics approach [23]. It was also demonstrated that the orientation of the anisotropic colloids can be described using the von Mises probability distribution.…”