Anisotropic Characteristic Lengths of Colloidal Monolayers Near a Water-Air Interface

TL;DR

This study investigates the anisotropic hydrodynamic behavior of colloidal monolayers near a water-air interface, revealing different characteristic lengths in perpendicular directions due to system discreteness and continuity.

Contribution

It introduces the concept of anisotropic characteristic lengths in colloidal monolayers and links them to the system's continuous and discrete nature in different directions.

Findings

X1 equals the Saffman length, indicating continuum behavior in the longitudinal direction.

X2 depends on Saffman length and colloid radius, indicating discrete effects transversely.

Hydrodynamics differ intrinsically between the two directions in the system.

Abstract

Near-interface colloidal monolayers have often been used as model systems for research on hydrodynamics in biophysics and microfluidic systems. Using optical microscopy and multiparticle tracking techniques, the correlated diffusion of particles is experimentally measured in colloidal monolayers near a water-air interface. It is found that the characteristic lengths X1 and X2 of such a colloidal monolayer are anisotropic in these two perpendicular directions. The former (X1)is equal to the Saffman length of the monolayer and reflects the continuous nature of the system in the longitudinal direction. The latter (X2)is a function of both the Saffman length and the radius of the colloids and reflects the discrete nature of the system in the transverse direction. This discovery demonstrates that the hydrodynamics intrinsically follow different rules in these two directions in this system.