The experimental realization of a two-dimensional colloidal model system

TL;DR

This paper details an experimental setup using colloidal particles at a water-air interface to study 2D phase transitions and glass transition, with precise control and observation of particle behavior.

Contribution

It introduces a novel experimental method for realizing and controlling a 2D colloidal system suitable for studying phase transitions and glass formation.

Findings

System can crystallize or form a glass depending on particle composition.

High-resolution video microscopy tracks thousands of particles.

Precise control of system parameters enables detailed phase transition studies.

Abstract

We present the technical details of an experimental method to realize a model system for 2D phase transitions and the glass transition. The system consists of several hundred thousand colloidal super-paramagnetic particles confined by gravity at a flat water-air interface of a pending water droplet where they are subjected to Brownian motion. The dipolar pair potential and therefore the system temperature is not only known precisely but also directly and instantaneously controllable via an external magnetic field B. In case of a one component system of monodisperse particles the system can crystallize upon application of B whereas in a two component system it undergoes a glass transition. Up to 10000 particles are observed by video microscopy and image processing provides their trajectories on all relative length and time scales. The position of the interface is actively regulated thereby reducing surface fluctuations to less than one micron and the setup inclination is controlled to an accuracy of 1 microrad. The sample quality being necessary to enable the experimental investigation of the 2D melting scenario, 2D crystallization, and the 2D glass transition, is discussed.