|
Colloidal particles in suspension provide versatile model systems for studying basic questions in condensed matter physics. Due to their intrinsic length and time scales in the micrometer and millisecond range they are readily accessible for measurements by means of light scattering or video microscopy, the particle density can easily be varied over orders of magnitude, and also other relevant parameters can be tuned, in particular the interaction energy between the particles, which can range from values much smaller to values much larger than kT.
We report here on experiments with spherical colloidal particles which form 2-dimensional systems at a substrate-water interface, and which are in addition locally confined by geometrical hard walls or by light corrals. The positions of the particles are recorded by a CCD camera, and the trajectories obtained from these measurements provide all the essential information about statics and dynamics. In this way phases and phase transitions of such systems and also the response to a rapid quench can be studied in detail. It is found that phase transitions in 2-dimensional confined particle ensembles or "clusters" exhibit a rich phase behavior, for which \x{2013} like in atomic clusters \x{2013} each particle counts [1]. In periodic pinning arrays, created by a holographic optical tweezers technique, transitions from incommensurate to commensurate solids and inhomogeneous melting is observed [2]. Transitions between different particle configurations have also been studied for the non-equilibrium case of particle transport in a channel. For sufficiently high particle interaction the formation of lanes is observed, which display abrupt structural changes in the presence of a density gradient along the channel [3]. [1] R. Bubeck, C. Bechinger, S. Neser, and P. Leiderer: "Melting and Reentrant Freezing of Two-Dimensional Colloidal Crystals in Confined Geometry", Phys. Rev. Lett. 82 (1999), p. 3364 [2] K. Mangold, P. Leiderer, C. Bechinger: "Phase Transitions of colloidal monolayers in periodic pinning arrays", Phys. Rev. Lett., 90, 158302 (2003) [3] M. Köppl, P. Henseler, A. Erbe, P. Nielaba, P. Leiderer: "Layer reduction in driven 2d-colloidal systems through microchannels", to appear in Phys. Rev. Lett. |
|