Viscoelastic drops moving on hydrophilic and superhydrophobic surfaces

Abstract

So-called "superhydrophobic" surfaces are strongly non-wetting such that fluid droplets very easily roll off when the surface is tilted. Our interest here is in understanding if this is also true, all else held equal, for viscoelastic fluid drops. We study the movement of Newtonian and well-characterised constant-viscosity elastic liquids when various surfaces, including hydrophilic (smooth glass), weakly hydrophobic (embossed polycarbonate) and superhydrophobic surfaces (embossed PTFE), are impulsively tilted. Digital imaging is used to record the motion and extract drop velocity. Optical and SEM imaging is used to probe the surfaces. In comparison with "equivalent" Newtonian fluids (same viscosity, density surface tension and contact angles), profound differences for the elastic fluids are only observed on the superhydrophobic surfaces: the elastic drops slide at a significantly reduced rate and complex branch-like patterns are left on the surface by the drop's wake including, on various scales, beads-on-a-string-like phenomena. The strong viscoelastic effect is caused by stretching filaments of fluid from isolated islands, residing at pinning sites on the surface pillars, of order ∼30 µm in size. On this scale, the local strain rates are sufficient to extend the polymer chains, locally increasing the extensional viscosity of the solution, retarding the drop.

Oscillatory Motion of Viscoelastic Drops on Slippery Lubricated Surfaces

“…2. More in detail, the bulge starts from the rear contact point of the polymeric drop where it is known that polymer feedback stresses take place [28][29][30] . Once the bulge is formed, the oscillating time law is simply a consequence of the periodic conversion between kinetic and gravitational potential energy of the deformed drop, as con rmed by solving the motion of a rigid wheel with a point mass xed on its boundary.…”

“…In addition to a proper rheology of the liquid drop, a key ingredient in our nding is a low contact line pinning, best achieved by impregnating a solid rough surface with a liquid lubricant. Interestingly, superhydrophobic dry rough surfaces, which are considered excellent water repellant, exhibit signi cant contact line pinning of elastic drops, resulting in enhanced friction 29,30 . Thus, adding a lubricant liquid makes the surface more similar to an ideal Young surface, perfect to address the dynamics of viscoelastic drops.…”

“…However, at variance with solutions containing stiff polymers, drops of solutions containing exible polymers exhibit a remarkable elongation in steady motion 28 . Drops of elastic liquids presenting constant viscosity, also known as Boger uids, moving on superhydrophobic and hydrophilic glass surfaces have also been investigated 29,30 . Boger uid drops, with very high elasticity but otherwise identical properties to Newtonian ones, show no difference compared to Newtonian uid drops while moving on hydrophilic glass surfaces.…”

Which Is the Motion State of a Droplet on an Inclined Hydrophilic Rough Surface in Gravity: Pinned or Sliding?

“…[15][16][17]. The study of the motion states of droplets on an inclined, hydrophilic rough surface in gravity is a fundamental problem in the mechanics of wetting and spreading [18][19][20][21][22][23][24][25], which facilitates a better understanding of how to manipulate a droplet on a rough surface. Obviously and simply, a small droplet on an inclined hydrophilic rough surface has two main motion states: pinned and inchworm sliding.…”

A Computational Model for the Analysis of Spreading of Viscoelastic Droplets over Flat Surfaces

“…The spreading of viscous and viscoelastic droplets over surfaces is an immensely important subject that has been studied extensively [1][2][3][4]. The detailed computational description is inherently an extremely challenging problem as it involves complex flow fields, movement and deformation of the droplet interface as well as creation of a new interface and these processes alter the boundary to the computational domain.…”