Active uniaxially ordered suspensions on disordered substrates

TL;DR

This paper investigates how quenched orientational disorder in substrates affects the phase behavior of active suspensions, revealing that polar suspensions maintain long-range order while apolar ones exhibit only short-range order with enhanced fluctuations.

Contribution

It introduces the effects of quenched substrate disorder on active suspensions, showing distinct behaviors for polar and apolar phases not previously detailed.

Findings

Polar suspensions exhibit long-range order in 2D with anomalous fluctuations.

Apolar suspensions have short-range order with activity-dependent correlation length.

Number fluctuations are more violent in active nematics with quenched disorder.

Abstract

Multiple experiments on active systems consider oriented active suspensions on substrates or in chambers confined along one direction. The theories of polar and apolar phases in such geometries were considered in A. Maitra et al, arXiv 1901.01069 and A. Maitra et al., PNAS 115, 6934 (2018) respectively. However, the presence of quenched random disorder due to the substrate cannot be completely eliminated in many experimental contexts possibly masking the predictions from those theories. In this paper, I consider the effect of quenched orientational disorder on the phase behaviour of both polar and apolar suspensions on substrates. I show that polar suspensions have long-range order in two dimensions with anomalous number fluctuations, while their apolar counterparts have only short-range order, albeit with a correlation length that can increase with activity, and even more violent number fluctuations than active nematics without quenched disorder. These results should be of value in interpreting experiments on active suspensions on substrates with random disorder.