The deposition behaviour of uniformly sized silica particles in drying aqueous droplets has been investigated for a range of particle sizes, 0.33, 1, 3 and 5 m, in order to gain an improved understanding of the coffee drop effect. The droplets were produced by inkjet printing, which allowed multiple droplets of similar volume to be studied. Our observations show that particle size and the contact angle formed by the solvent droplet with the substrate determine how close to the boundary a particle is deposited. After drying, it was found that if the contact angle was less than 90°, smaller particles were located closer to the original droplet's periphery than larger particles in similar sized droplets. This deposition of particles can be explained by the wedge shape of the drying droplet's edge, which physically limits the movement of particles towards the droplet's periphery. In this paper we show that the size of a suspended particle influences the final dried morphology of a printed feature.
Perelaer, J., Smith, P. J., Hendriks, C. E., Berg, van den, A. M. J., & Schubert, U. S. (2008). The preferential deposition of silica micro-particles at the boundary of inkjet printed droplets. Soft Matter, 4(5), 1072-1078. https://doi.org/10.1039/b715076h