Surfactant Effects on the Dynamics of Capillary Rise and Finger Formation in Square Capillaries

We
investigate the influence of surfactants on capillary rise and
corner flow in angular pores. We therefore study capillary rise for
simple fluids and surfactant solutions, comparing square to cylindrical
capillaries. We show that fingers start to form in the corners of
the square capillaries when the capillary rise slows down before reaching
the equilibrium height. The corner flow scales as t1/3 and its quantitative understanding necessitates that
the surface wettability is taken into account in terms of the liquid’s
advancing contact angle on the capillary walls inside the corner.
Adding surfactants to water greatly influences the corner flow in
square capillaries: depending on the nature of the surfactant, the
corner flow can be either suppressed completely due to autophobic
effects or enhanced due to the presence of Marangoni stresses caused
by a surface tension gradient inside the liquid fingers.