Thermophoresis & Soret Effect

Temperature gradient ∇T: 50 K/mm

Particle size r: 200 nm

Soret coefficient S_T: +0.010 K⁻¹

▶ Toward cold (S_T > 0)

◀ Toward hot (S_T < 0)

+12.5

Drift v (nm/s)

0.73

Soret separation

Thermophoresis

Colloidal particles in a temperature gradient experience a net force due to asymmetric molecular bombardment. The particle flux is:

J = −D_T · c · ∇T = −S_T · D · c · ∇T

At steady state, diffusion balances thermophoresis (Soret equilibrium):

c(x)/c₀ = exp(−S_T · ΔT)

Most particles have S_T > 0 (move toward cold). Some polymers and DNA at high ionic strength show negative thermophoresis (toward hot side).

Thermophoresis is used in microscale trapping (laser heating), protein concentration assays (MST), and is thought to play a role in the origin of life in hydrothermal pores.