Desorption kinetics during capillary flow

A model for coupling desorption and convective diffusion processes during capillary flow is considered. The desorption leads to the decrease in surface concentration, i.e., to surface cleaning. The model is specified with respect to the cleaning of long, narrow tubes of endoscopes, specifically for the case when desorption plays a larger role than hydrodynamic detachment. During the final stage of cleaning, coverage of the tube surface by contaminants (adsorbate) is low. Consideration of contaminants as isolated adsorbed particles (molecules) with no lateral interaction with each other on a uniform tube surface enables one to use a rather general mixed-desorption model called the kinetic-diffusion controlled model. Our model predicts that after cleaning, the tube remains slightly contaminated near its exit even though there is almost complete cleaning at the entrance. This is caused by regularities of the coupling between desorption and convective diffusion processes. Since this prediction agrees with our observations, a practical recommendation for improving tube exit cleaning is formulated. Modeling of this system became possible due to the simple velocity distribution of Poiseuille's flow in this geometry, which allows us to apply the Levich theory of convective diffusion in our modeling.