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HERE FOR FULL-SIZE VIDEO (14MB) AND ELASTIC FLUIDS D. C. Roux, University of Melbourne and Universite Joseph Fourier, J. J. Cooper-White, University of Melbourne, G. H. McKinley, Massachusetts Institute of Technology, and V. Tirtaatmadja, University of Melbourne This video presents a study of the dynamics of drop impact of a Newtonian fluid (water) and a constant viscosity elastic fluid (Boger fluid) of matched shear viscosities (~1.0-1.3 mPa·s) impacting on solid hydrophilic and hydrophobic surfaces and on a thin liquid film (~1 mm in depth) of the same fluid. A high speed video camera and a beam splitter cube allows us to see both the bottom and side views of the drop impact simultaneously with a time interval between each image of Δt =0.526 ms and an exposure time of Δtexp=10 µs. On the hydrophilic surface the spreading of the two solutions occurs over the same time scale,1 giving a flat internal disk surface with a peripheral thick annulus but with different amplitude and frequency of fingers instabilities. During drop recoil, the capillary waves are strongly dampened in the case of the elastic solution.2 The Newtonian fluid gradually assumes an equilibrium drop profile, retracting from the hydrophilic surface slowly, while the elastic fluid does not retract to any significant extent during the same period of time. The elastic fluid is observed to be more wetting than water itself on this hydrophilic surface (glass). This is believed to be due to the rapid adsorption of the polymer to the surface. On the hydrophobic surface, the spreading of both solutions is similar to that observed on the hydrophilic surface. However, the dynamics of recoil illustrate completely different behavior.3 The peripheral fingers of the Newtonian solution grow by coalescence to form star-like arms during the recoil of the fluid. The strong inertial flow caused by the rapid recoil of the drop results in the whole drop leaving the surface. For the elastic drop, the recoil of the solution is retarded. Extension of the polymer in this biaxial extensional flow consumes much of the available energy for recoil3; however, the adsorption of the high molecular weight polymer to the surface is believed to further reduce the energy for the recoil and suppress the rebound. Peripheral fingers adhered to the surface undergoing film breakage as the bulk of the drop recoils from the solid surface. This film breakage event generates secondary capillary waves on the free surface. When both solutions impact a thin liquid film of the same fluid, a liquid sheet terminated by a corolla is produced. For the Newtonian solution, corolla jetting at the periphery occurs immediately, ejecting many small individual drops above the film, creating a crown. Thereafter, once the maximum height is achieved, the crown collapses due to gravity and surface tension. Surface tension causes breakup of the jetting fingers into drops. The liquid sheet of the elastic solution initially grows with a similar velocity but jetting is significantly retarded, with the evolution of only small bulbs of fluid at the upper peripheral ring. When the crown breaks down, the bulbs of the fluid on the periphery remain attached to the liquid sheet. Due to the balance between surface tension and elasticity, thin extended threads are created joining the bulbs and the liquid sheet. When the surface tension energy exceeds the elastic energy, secondary drops are formed along the thread. After the drops lose their kinetic energy, they collapse onto the fluid film carrying these elongated threads with them. This inhibited breakup of the threads and reduction in secondary spray is important in enhancing the delivery and deposition efficiency of complex fluids such as fertilizers. 1
I. V. Roisman, R. Rioboo, and C. Tropea, Proc. R. Soc. London, Ser. A
458, 1411 (2002). |
