(447b) The Effect of Marangoni Currents and Gravitational Forces on the Separation of Two Different Sized Particles during Droplet Evaporation

When a homogenously mixed droplet with two different sized particles evaporates, the particles can separate by size into different rings with one ring for each particle size. This work shows that particle separation is significantly affected by Marangoni currents and by gravitation forces. The finite element method was used to create a transport model that tracks each particle concentration, fluid velocity, pressure, temperature, and vapor concentration. The model created is two dimensional with an axisymmetric assumption. The computational model shows that a majority of particles remain suspended in the droplet due to Marangoni currents. This is in contrast to previously published isothermal models which showed particles rapidly accumulating at the contact line. Experiments using 1 μm and 3 μm polystyrene particles support the computational results. Additional experiments with pendant and sessile droplets show that previous assumptions to neglect gravitational effects on 1 μm polystyrene spheres are incorrect. Finally, the importance of both thermal effects and gravitational forces on the separation of two different sized particles into rings is shown. This is achieved by comparing the depositions profiles for 1 μm and 3 μm particles with and without thermal and gravitational effects.