Thermocapillary

Thermocapillary, or the thermal Marangoni effect, describes fluid motion driven by gradients in surface tension across a liquid interface caused by temperature variations. The effect arises at liquid–gas or liquid–liquid boundaries and is prominent when interfacial forces dominate viscous forces, such as in thin films, droplets, and microfluidic channels.

Surface tension of most liquids decreases with increasing temperature. A lateral temperature gradient along an interface creates a gradient in surface tension, producing a tangential stress (Marangoni stress) that drives fluid from regions of low surface tension to regions of high surface tension. This generates interfacial flows, internal circulations within droplets, coating flows, and, in some cases, self-propulsion of droplets along the gradient. The flow pattern depends on geometry, boundary conditions, and whether the system is gravity- or confinement-dominated.

Governing descriptions often use the Marangoni number Ma, which relates interfacial tension forces to viscous and diffusive transport: Ma = (d sigma/dT) ΔT L /(mu alpha). Larger Ma indicates stronger thermocapillary effects and can lead to instabilities such as surface fingering in thin films.

Applications and occurrences include droplet microfluidics (pumping and mixing without external pumps), coating flows and thin-film processing, solder reflow and crystal growth, and convection in heated liquid layers. Understanding thermocapillary phenomena is important in electronics manufacturing, materials processing, and microengineering, where precise control of interfacial flows is required.