Drop Shape Oscillations
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The present invited contribution to this book reviews the state of understanding and application of shape oscillations of liquid drops in a gaseous environment. The oscillations influence transport processes across the surface of spray drops, the drag as well as heat and mass transfer. The physical basics of linear and nonlinear oscillations are presented and discussed. For the linear case, the characteristic equation of the drop is derived, and the equations of motion are solved, accounting for the fact that the characteristic equation has pairs of complex conjugate solutions. The effects characterising the nonlinear case are reviewed and discussed. Shape oscillations of non-Newtonian, viscoelastic liquid drops exhibit interesting influences from time scales of the viscoelastic liquid relative to the oscillation period. The liquid elasticity may take over from surface tension as the restoring effect. Drop shape oscillations are used for measuring material properties of the drop liquid, such as dynamic viscosity and surface tension, as well as rheological and interfacial parameters. The most important measurement techniques and measured liquid properties are presented and discussed.
Support from the Deutsche Forschungsgemeinschaft (DFG) and the Austrian Science Fund (FWF) through the joint project I-3326-N32 is gratefully acknowledged.
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