Modeling of the heat transfer in laser-heated small particles on surfaces

Motivated by new photothermal techniques to detect trace quantities of illicit substances, we are examining the heat transfer of small particles randomly distributed on surfaces that are selectively heated by a laser beam. For an optimal choice of laser power, illumination time and other parameters, we need formulas describing how the thermal signal depends on particle size, their distribution on the substrate surface and thermo-physical properties of the materials involved. In this manuscript we compare very simple physical models and computational simulations with experimental data from polyethylene microspheres on a polished copper surface, with diameters ranging from 20 to 200 μm. Heat transfer through air dominates the process, both for single particles and between particles in clusters. The influence of high particle densities per substrate area on the heat transfer process is factored into the simulation by using a cell with just one particle and symmetric boundary conditions. Further simulations where irregular-shaped carbon pieces were approximated as lying cylinders indicate that our simple model can also describe the thermal behavior of a wider class of realistic particles on solid surfaces