Trapping colloids near chemical stripes via critical Casimir forces

TL;DR

This paper investigates how critical Casimir forces near chemically patterned substrates can trap and control colloids, with theoretical and experimental evidence showing tunable, temperature-dependent effective potentials.

Contribution

It introduces a combined theoretical and experimental study of critical Casimir forces on colloids near patterned substrates, demonstrating tunable trapping mechanisms.

Findings

Effective lateral confinement of colloids observed near striped patterns.

Measurements agree with theoretical predictions across all substrates.

Temperature changes can tune the strength and direction of forces.

Abstract

We study theoretically and experimentally the solvent-mediated critical Casimir force acting on colloidal particles immersed in a binary liquid mixture of water and 2,6-lutidine and close to substrates which are chemically patterned with periodically alternating stripes of antagonistic adsorption preferences. These patterns are experimentally realized via microcontact printing. Upon approaching the critical demixing point of the solvent, normal and lateral critical Casimir forces generate laterally confining effective potentials for the colloids. We analyze in detail the rich behavior of the spherical colloids close to such substrates. For all patterned substrates we investigated, our measurements of these effective potentials agree with the corresponding theoretical predictions. Since both the directions and the strengths of the critical Casimir forces can be tuned by minute temperature changes, this provides a new mechanism for controlling colloids as model systems, opening encouraging perspectives for applications.