Direct measurement of thermophoretic forces

Area of Science:

• Colloid science
• Soft matter physics
• Thermodynamics

Background:

• Thermophoresis describes particle movement in response to temperature gradients.
• Understanding thermophoresis is crucial for manipulating particles in various applications.
• Existing methods have limitations for certain particle sizes and solvent combinations.

Purpose of the Study:

• To develop and validate a new method for quantifying thermophoretic forces on micron-sized colloidal particles.
• To determine the temperature dependence of the Soret coefficient (ST(T)) for specific particle types.
• To extend the applicability of thermophoresis measurements to previously inaccessible systems.

Main Methods:

• Utilizing evanescent light scattering to study single colloidal particle motion.
• Analyzing the nonequilibrium steady state (NESS) probability distribution to derive thermophoretic forces via a generalized potential.
• Experimentally measuring spatial probability distributions with high force resolution (10 fN).

Main Results:

• Successfully extracted thermophoretic forces from particle position probability distributions.
• Determined the temperature dependence of the Soret coefficient (ST(T)) for polystyrene and melamine particles.
• Observed good agreement between experimental results and theoretical predictions for ST(T).

Conclusions:

• The proposed generalized potential method offers a robust way to quantify thermophoretic forces.
• This technique enables thermophoresis measurements for larger particles and diverse solvent systems.
• The findings contribute to a deeper understanding of colloidal particle behavior in temperature gradients.