Knudsen-to-Hydrodynamic Crossover in Liquid 3He in a High-Porosity Aerogel

TL;DR

This study investigates the transition from Knudsen to hydrodynamic regimes in liquid helium-3 within a high-porosity aerogel, revealing a clear crossover in sound attenuation behavior driven by quasiparticle interactions.

Contribution

The paper combines experimental measurements and theoretical modeling to demonstrate the Knudsen-hydrodynamic crossover in liquid ${}^3$He in aerogel, with detailed analysis of sound damping laws.

Findings

Clear evidence of Knudsen-hydrodynamic crossover in ultrasound attenuation

Distinct laws governing sound damping in two regimes

Excellent agreement between theory and experiment

Abstract

We present a combined experimental and theoretical study of the drag force acting on a high porosity aerogel immersed in liquid ${}^3$He and its effect on sound propagation. The drag force is characterized by the Knudsen number, which is defined as the ratio of the quasiparticle mean free path to the radius of an aerogel strand. Evidence of the Knudsen-hydrodynamic crossover is clearly demonstrated by a drastic change in the temperature dependence of ultrasound attenuation in 98% porosity aerogel. Our theoretical analysis shows that the frictional sound damping caused by the drag force is governed by distinct laws in the two regimes, providing excellent agreement with the experimental observation.