Contact angle hysteresis on dentin surfaces measured with ADSA on drops and bubbles

Abstract

Contact angle measurement on biological surfaces is difficult due to substrate hydration, porosity and heterogeneity. The former promotes very low contact angles, and the latter leads to irregular drop shapes and contact angle hysteresis, being meaningful in terms of surface energetics only low rate dynamic contact angles, i.e. the extreme angles corresponding to a slowly advancing and receding contact line. ADSA-CD has been developed to take into account the drop shape irregularity. It requires as input the liquid surface tension and density, the co-ordinates of some contact line points and the drop volume, yielding a mean contact angle. Due to the latter requirement ADSA-CD has been used until now only for static contact angle measurements, i.e. at constant drop volumes. We present an upgrade of the method that permits to control very precisely the drop volume, and to capture simultaneously drop images for contact angle determination, permitting this way to use ADSA-CD for hysteresis measurements. Low contact angles lead to difficulties in their measurement, being the captive bubble technique a known method to overcome this shortcoming. As our system is also capable to measure captive bubble low rate dynamic contact angles, we performed an experimental study with Teflon and bitumen model surfaces, which permits to identify the advancing bubble angles with the receding drop angles. We were therefore able to measure hysteresis on dentin tissue, which has a too low receding drop angle to be measured by ADSA-CD.