8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P4.53Controlling droplet impact with polymer additivesMichael Smith 1 and Volfango Bertola 21 University of Nottingham, School of Physics, University of Nottingham NG7 2RD,Nottingham, United Kingdom2 University of Edinburgh, Edinburgh, United KingdomControlling droplet impact is of tremendous commercial and environmental importance. When awater drop falls on to a hydrophobic surface, such as the waxy leaf of a plant, the drop is oftenobserved to bounce off leading to wasted agrochemicals which harm the environment. However,for about 10 years it has been known that the addition of small quantities (∼100 µgml −1 ) of aflexible polymer such as poly-(ethylene oxide) can completely prevent rebound. This is surprisingsince the shear viscosity and surface tension of such drops are almost identical to those of purewater. The effect has for some time been explained in terms of the stretching of polymer chainsby a velocity gradient in the fluid, resulting in a transient increase in the so-called ”extensionalviscosity”. We have developed an epi-fluorescent microscope system, complete with a pulsedlaser and high speed camera which enabled visualisation of the flow of fluid inside an impactingdrop using fluorescent tracer particles at 2000 frames/s. Analysis of the velocity as a functionof radius showed negligible differences between pure water and PEO drops except near the dropedge, indicating that the extensional viscosity cannot be responsible for the anti-rebound effect.To probe the true mechanism, fluorescently labelled λ-DNA in PEO drops was used to visualisethe edge of an impacting drop. During drop impact, in the retraction phase, DNA was shown tobe stretched by the retreating droplet providing an ”effective friction” at the contact line. A betterunderstanding of the mechanism responsible for the rebound suppression will aid in the designof more effective additives, for the agrochemical industry but also in other important commercialapplications such as inkjet printing and spray cooling. [1] M. I Smith and V. Bertola, Phys. Rev.Letts. 104, 154502 (2010).53