Insights into the static and advancing water contact angles on surfaces anisotropised with aligned fibers: Experiments and modeling

We report the experimental and modeling results of water contact angle (CA) on anisotropic surfaces with aligned fibers fabricated by electrospinning. The aligned fibers with radius of several micrometers can effectively stop the three-phase contact line (TPCL) and lead to a very high static CA as well as stick-slip patterns. The maximal CA, which appears during the advancing CA measurement, is nearly the same on those surfaces with different fiber densities. A thermodynamic model was applied to describe the free energy change and the location of metastable states. It is found that the energy barrier of potential wells is precisely linear to the fiber radius. By adjusting the reference energy of the model, the calculated result can fit the experimental ones well. Results also indicate that the process for static CA measurement is similar to that for advancing CA to some extent but without much driving force. Static CA in the direction parallel to the fibers is very different from that perpendicular to the fibers since there is no obvious barrier for the TPCL to advance along the fibers. These experimental and modeling results provide some insights into the CA on rough surfaces as well as the stick-slip phenomenon. Relationship between the metastable potential wells and the external energy determines when the slip or stick happens. Furthermore, the results may be helpful for designing of microfluidic system. (C) 2011 Elsevier B.V. All rights reserved.