Controlling droplet deposition with polymer additives

Controlling the impact of drops onto solid surfaces is important for a wide variety of coating and deposition processes-for example, the treatment of plants with herbicides and pesticides requires precise targeting in order t o meet stringent toxicological regulations. However, the outer wax-like lay er of the leaves is a non-wetting substrate that causes sprayed droplets to rebound; often less than 50% of the initial spray is retained by the plant (1). Although the impact and subsequent retraction of non-wetting aqueous d rops on a hydrophobic surface have been the subjects of extensive experimen tal and theoretical work(2-7), non-newtonian rheological effects have not b een considered in any detail. Here we report that, by adding very small amo unts of a flexible polymer to the aqueous phase, we can inhibit droplet reb ound on a hydrophobic surface and markedly improve deposition without signi ficantly altering the shear viscosity of the solutions. Our results can be understood by taking into account the non-newtonian elongational viscosity, which provides a large resistance to drop retraction after impact, thereby suppressing droplet rebound.