Controlling droplet deposition with polymer additives

Abstract

Controlling the impact of drops onto solid surfaces is important for a wide variey of coating and deposition processes--for example, the treatment of plants with herbicides and pesticides requires precise targeting in order to meet stringent toxicological regulations. However, the outer wax-like layer 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. Although the impact and subsequent retraction of non-wetting aqueous drops on a hydrophobic surface have been the subjects of extensive experimental and theoretical work, non-newtonian rheological effects have not been considered in any detail. Here we report that, by adding very small amounts of a flexible polymer to the aqueous phase, we can inhibit droplet rebound on a hydrophobic surface and markedly improve deposition without significantly 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.

Retraction dynamics of aqueous drops upon impact on non-wetting surfaces

“…However, due to the hydrophobicity of the surface, subsequently the drop retracts violently, leading to the ejection of part of the droplet from the surface: we observe droplet rebound. It is this "rebound" that is the limiting factor for deposition in many applications, for instance for the deposition of pesticide solutions on hydrophobic plant leaves (Bergeron et al 2000). We study here the impact and subsequent retraction of aqueous drops onto a hydrophobic surface, and seek to understand the dynamics of expansion and retraction of the droplets.…”

Short time dynamics of viscous drop spreading

“…This process is relevant for many applications, from printing and coating to agriculture [1][2][3][4][5] .…”

Droplet Detachment and Satellite Bead Formation in Viscoelastic Fluids

“…From the fundamental side, the addition of minute amounts of polymers has been shown to inhibit the finite-time singularity that happens at breakup [5]. From the applied side, in many applications, such as fire fighting [6], ink jet printing [3], or pesticide deposition on plant leaves [7], complex fluids have been used to control or modify drop sizes. All these applications are related to the elevated elongational viscosity of polymer solutions.…”