Effect of polydispersity on the behaviour of the particle-laden interface

Abstract

This paper studies the effect of polydispersity on the behaviour of micron size particles spread at the air–water interface using a modified Langmuir trough. Surface pressure isotherms of particles with different polydispersity prepared by mixing large and small particles in various ratios were studied. Upon lateral compression, the surface pressure of the monolayer consisting of the unimodal samples was found to increase faster than the surface pressure of the layer formed with the bimodal samples. Compared to the unimodal samples, the bimodal sample monolayers had higher compressibility at the highest surface pressure. The results showed an increasingly loose structure and smaller domains with an increasing ratio of large particles in the monolayer. A single bubble beneath the particle layer was grown and allowed to coalesce with the layer. The structure and motion of particles following the coalescence process was recorded by a high-speed camera. A mean square displacement was calculated from the trajectory of selected particles to evaluate the movement of these particles during the coalescence of the bubble with the particle layer. The peak of mean square displacement was higher for low packing density and lower at high packing density, indicating the particles have more movement at low packing density than at high packing density. It was also shown that the particles became more mobile with an increase in the proportion of large particles in the layer.