Abstract. -We experimentally determine effective interparticle potentials in a two-dimensional (2D) colloidal system of charge-stabilized polystyrene particles at different particle densities ρ. Density variation is achieved by means of a scanned optical laser tweezer which serves to create a boundary box for the system. By changing the size of this boundary box, ρ can be systematically varied without having to prepare a new system. From the measured radial distribution functions we can then obtain the effective pair potentials of the particles. While for low particle densities perfect agreement with Yukawa-like potentials is observed, considerable deviations from this form are found at higher densities. We interpret this result as a many-body effect produced by macroion screening which is expected to become more pronounced as the density is increased.There are several analogies between a charge-stabilized colloidal suspension and a fluid metal. A metal essentially consists of highly charged core ions plus the screening charge cloud of the electrons, which the ions are embedded in [1]. Colloidal suspensions, on the other hand, are composed of large and highly charged particles, i.e. macroions which are suspended in a structureless medium and surrounded by a screening atmosphere of microions. Common to both systems is that there are two classes of ingredients (ions/electrons and macroions/microions) which move and respond on totally separated length and time scales. Accordingly, similar theoretical concepts can be applied in both the theory of metals and that of colloidal suspensions. One of the major theoretical tasks in the description of metals is to reduce the many-body electron-ion Hamiltonian to an effective ionic Hamiltonian which is expressed as a sum of volume-, pair-, triple-, and multi-ion interactions. This can be achieved by eliminating the electronic degrees of freedom. In a similar manner, one of the fundamental questions in colloidal systems is, how to integrate out the microionic degrees of freedom and to calculate effective macroion/macroion forces, consisting of the direct Coulomb repulsions and an indirect interaction mediated by the small ions of the electrolyte. In contrast to fluid metal, however, a suspension of charged colloidal particles can directly be observed and studied under an optical microscope. Accordingly, colloidal suspensions provide an ideal testing ground for the various concepts to attack the interesting many-body problem. In the following, we will in particular focus on the widely used concept of "effective" pair interactions [2,3].
The nature of freezing and melting transitions for a system of hard disks in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analysis of various thermodynamic quantities like the order parameter, its cumulants, etc., are used to map the phase diagram of the system for various values of the density and the amplitude of the external potential. We find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of hard disks behaves in a fashion similar to charge-stabilized colloids that are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. Detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting.
The nature of freezing and melting transitions for a system of model colloids interacting via the Derjaguin, Landau, Verwey, and Overbeek potential in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analyses of various thermodynamic quantities, such as the order parameter, its cumulants, etc., are used to map the phase diagram of the system for various values of the reduced screening length kappaa(s) and the amplitude of the external potential. We find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of soft disks behaves in a fashion similar to charge stabilized colloids, which are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. The detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting.
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