A molecular dynamics study of photothermal compression of colloidal crystals

When irradiated with a laser, the lattice constant of a colloid crystal changes in response to the temperature gradient created in the solvent. Using constant pressure/constant temperature molecular dynamics simulations, the expansion and compression of charge stabilized colloid crystals is investigated. Two systems are considered, both of which correspond approximately to samples which have been experimentally characterized. It is shown that these colloid crystals do not melt over a temperature range of 298 K to 368 K. One system only expands with increasing temperature, while the other initially expands and then contracts. Colloid number density is calculated as a function of temperature at the center of the heated region. An analytical model of the dependence of equilibrium lattice parameter on external pressure for a crystal characterized by zero colloid temperature is presented. Because these crystals remain relatively rigid even in the physically relevant temperature range, it is argued that a zero colloid temperature model should be qualitatively reasonable. Indeed, the model calculations support the basic conclusions drawn from full molecular dynamics simulations, and by comparison highlight effects due to finite temperature motion of the colloid particles.