Synopsis: How Fluids Flow When the Temperature Changes

Physicists develop a theory to make the seemingly random, thermally driven motion of particles in fluids predictable.

When smoke particles drift from a fire, they move because of a force that arises from the presence of a thermal gradient in the air. However, different particles in the same thermal gradient can follow different paths: Some move toward hot patches, while others move toward cold spots. Experts do not fully understand the microscopic physics that dictates this motion, especially in liquids. Now, Alberto Parola and colleagues at the University of Insubria in Italy have developed a theory that describes this thermal-gradient-driven motion in liquid and gas, known as thermo-osmosis.

To develop their theory, the team modeled this phenomenon in its simplest geometry: a temperature gradient in a fluid near a flat surface. The temperature gradient causes particles in the fluid to move. But the paths of the particles also depend on other system properties, including the viscosity of the fluid and the local pressure. The paths further depend on how far the particles are from the surface: Closer particles tend to hit the flat surface and exchange momentum and energy, which alters their trajectories.

The team’s theory is consistent with prior, more idealized models of thermo-osmosis, including one formulated by James Clerk Maxwell in the 19th century that describes the motion of gas far from a surface. The new theory applies to both liquid and gas phases, and the team says that it could be used to study particle motion in fluids contained inside differently shaped surfaces, such as a sphere or a sealed tube.

This research is published in Physical Review Letters.

–Sophia Chen

Sophia Chen is a freelance science writer based in Tucson, Arizona.


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