Abstract
We present thermophoretic measurements in aqueous suspensions of three different polystyrene (PS) particles of varying negative charge, size, and surface coating. Our measurement technique is based on the observation of the colloidal steady-state distribution using conventional bright-field microscopy, which avoids undesirable effects such as laser-induced convection or local heating. We find that the colloids with the weakest zeta potential exhibit the strongest thermophoretic effect, suggesting that the Soret coefficient has a more intricate dependence on surface functionality than predicted by existing theoretical approaches. We also study the relaxation of the colloids to steady-state and propose a model to quantify the relaxation speed, based on the time evolution of the colloidal center of mass. Our observations are well described by this model and show that the relaxation speed tends to increase with the magnitude of the thermophoretic force.
Highlights
The motion of particles in a temperature gradient is known as thermophoresis
We have introduced a measurement technique for thermophoresis that is based on observing the change of the colloidal steady-state concentration profile with the applied temperature gradient
Our measurements show that the Soret coefficient is rather insensitive to temperature for charged PS particles in aqueous suspensions
Summary
The motion of particles in a temperature gradient is known as thermophoresis. The motion of colloids resulting from thermodynamic gradients in concentration and temperature is quantified by the total particle flux J, which is given by. The effective force FT that drives thermophoresis can be written as FT = γvT = −kBTST ∇T. Using this expression for FT , the steady-state concentration profile of colloids in a temperature gradient can be expressed as. (2) and (4), which show that the Soret coefficient ST can be determined from the colloidal concentration profile at steady-state when the temperature gradient is known Our experimental technique is based on Eqs. (2) and (4), which show that the Soret coefficient ST can be determined from the colloidal concentration profile at steady-state when the temperature gradient is known
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