A universal capillary-deflection based adhesion measurement technique.

Abstract

Contact angle goniometry suffers from inherent optical challenges such as scattering and diffraction near the triple contact line (TCL) rendering erroneous results. Alternatively, the cantilever-based direct adhesion measurement was constrained to low-energy surfaces to date due to the inability of the probe droplet to retract (pull-off) from high-energy surfaces completely. The present study revisits the cantilever approach from a fundamental physical perspective and generalizes the approach to render it wettability invariant. The adhesive wetting interaction between a probe droplet (attached to a cantilever) with the test substrate is recorded with a high-speed camera. Image processing and subsequent motion analysis enable us to accurately calculate the adhesion force (in the sub-micron range) exhibited by the test substrate. We experimentally demonstrate the contact line depinning (and not the droplet pull-off) to be the only prerequisite for accurately quantifying the characteristic adhesion force. We alsoreveal that depinning precedes the onset of cantilever retraction due to the inertia effect. Further, we establish that the characteristic adhesion corresponds to zero acceleration of the cantilever and not to its maximum deflection. The inferences of the study will be beneficial in the rational design of direct wetting characterization methods for any substrate.