Contact angles and wettability of cellulosic surfaces: a review of proposed mechanisms and tests strategies
Contact angle methods are widely used to evaluate the wettability of cellulose-based surfaces and to judge their suitability for different applications. Wettability affects ink receptivity, coating, absorbency, adhesion, and frictional properties. There has been a continuing quest on the part of researchers to quantify the thermodynamic work of adhesion between cellulosic surfaces and various probe liquids and to account for such components of force as the London/van der Waals dispersion force, hydrogen bonding, and acid and base interactions. However, due in part to the rough, porous, and water-swellable nature of cellulosic materials, poor fits between various theories and contact angle data have been observed. Such problems are compounded by inherent weaknesses and challenges of the theoretical approaches that have been employed up to this point. It appears that insufficient consideration has been given to the challenging nature of cellulosic materials from the perspective of attempting to gain accurate information about different contributions to surface free energy. Strong hysteresis effects, with large differences between advancing and receding contact angles, have been overlooked by many researchers in attempting to quantify the work of adhesion. Experimental and conceptual approaches are suggested as potential ways to achieve more reliable and useful results in future wettability studies of cellulosic surfaces.