Planar and Nematic Aerogels: DLCA and Superfluid 3He

TL;DR

This study uses cluster aggregation simulations to model anisotropic aerogels, classifies their structures via structure factor analysis, and discusses implications for superfluid helium-3 experiments.

Contribution

It introduces a simulation method to create and classify anisotropic aerogel structures and compares them with experimental data, revealing how compression and stretching affect aerogel morphology.

Findings

Simulated nematic aerogel matches lab-grown axially-compressed silica aerogel.

Simulated planar aerogel matches lab-grown stretched silica aerogel.

Compression and stretching induce long strands or planes in silica aerogel.

Abstract

We perform cluster aggregation simulations to model the structure of anisotropic aerogel. By biasing the diffusion process, we are able to obtain two distinct types of globally anisotropic aerogel structures which we call "nematic", with long strands along the anisotropy axis, and "planar", with long strands in planes perpendicular to the anisotropy axis. We calculate the auto-correlation function, the structure factor, and the angular dependence of the free-path distribution for these samples. The calculated structure factor from simulated aerogels can be compared with data from small-angle X-ray scattering (SAXS) of lab-grown aerogel allowing us to classify the spatial structure of the lab-grown samples. We find that the simulated "nematic" aerogel has a structure factor consistent with lab-grown, axially-compressed silica aerogel while the simulated "planar" aerogel has a structure factor consistent with lab-grown "stretched" silica aerogel. Unexpectedly, compressing previously isotropic silica aerogel leads to the formation of long strands along the compression axis while stretching silica aerogel leads to formation of planes perpendicular to the stretching axis. We discuss the implication of this determination on experiments of superfluid $^3$He in anisotropic aerogel, in particular the orbital analog of the spin-flop transition.