Height Distribution and Orientation of Colloidal Dumbbells Near a Wall

TL;DR

This study investigates how colloidal dumbbells behave near walls, revealing size-dependent orientations and forces, which enhances understanding of nonspherical particle dynamics in confined liquid environments.

Contribution

It provides the first detailed measurements of height distribution and orientation of colloidal dumbbells near walls, incorporating gravitational and electrostatic effects.

Findings

Larger dumbbells tend to align parallel to the wall.

Smaller dumbbells exhibit preferred orientations at specific angles.

Net forces and torques can be below thermal energy at certain heights.

Abstract

Geometric confinement strongly influences the behavior of microparticles in liquid environments. However, to date, nonspherical particle behaviors close to confining boundaries, even as simple as planar walls, remain largely unexplored. Here, we measure the height distribution and orientation of colloidal dumbbells above walls by means of digital in-line holographic microscopy. We find that while larger dumbbells are oriented almost parallel to the wall, smaller dumbbells of the same material are surprisingly oriented at preferred angles. We determine the total height-dependent force acting on the dumbbells by considering gravitational effects and electrostatic particle-wall interactions. Our modeling reveals that at specific heights both net forces and torques on the dumbbells are simultaneously below the thermal force and energy, respectively, which makes the observed orientations possible. Our results highlight the rich near-wall dynamics of nonspherical particles, and can further contribute to the development of quantitative frameworks for arbitrarily-shaped microparticle dynamics in confinement.