Effects of surface-active impurities on the liquid bridge dynamics

Abstract

We examine experimentally the effects of surface-active impurities on the small-amplitude free oscillations of axisymmetric liquid bridges. The surface tension, oscillation frequency, and damping rate are measured at different instants from the free surface formation. The experiments with n-hexadecane and n-dodecane show that none of these interfacial quantities is significantly affected by the free surface age. The damping rates exceed by a $$\mathcal{O}(1)$$ quantity their corresponding values for a clean free surface. This extra-damping can be modeled in terms of the monolayer shear viscosity exclusively. Similar values of this quantity are obtained for n-hexadecane and n-dodecane, although the impurity effects on the surface tension are much greater in the first case. We conducted experiments with deionized water liquid bridges to analyze the impurity effects on free surfaces with high elasticity numbers, where Marangoni convection is expected to increase the monolayer dissipation. In this case, the damping rates are up to three times as those of a clean free surface. The monolayer dissipative effects do not increase as the free surface ages, although the surface tension decreases considerably during this process. Similar impurity effects are observed when an anionic surfactant is dissolved in deionized water. For a fixed value of the liquid bridge slenderness, both the oscillation frequency and damping rate are functions of the liquid bridge volume exclusively, independently of the free surface age and the liquid-ambient combination. Extra-dissipation increases sharply as the liquid bridge volume decreases.