The effect of liquid properties to contact angle hysteresis

Low-rate dynamic advancing and receding contact angles of 21 liquids from t wo homologous series (i.e. n-alkanes and 1-alcohols) and octamethylcyclotet rasiloxane (OMCTS) on FC-732-coated silicon wafer surfaces were measured by an automated Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The recedi ng contact angle was found to decrease with time, suggesting surface swelli ng and sorption. An initial receding angle (theta (ri)), which theoreticall y represents the receding angle before sorption starts, was extrapolated ba ck to time of zero contact (theta (0)) by least-squares regression in each experiment. The contact angle hysteresis, i.e. the difference between advan cing contact angle and theta (ri), was found to decrease with increasing ch ain length of the liquid molecules for both the alkane and alcohol series. Contact angle hysteresis was found to vanish when the chain length was extr apolated to infinity. These results are plausible in terms of liquid sorpti on by the solid surface: very large molecules are unlikely to penetrate int o the solid film. In order to reconfirm that contact angle hysteresis depen ds on molecular size of the liquid, contact angles of OMCTS, which is a bul ky and quasi-spherical molecule, were measured. The hysteresis was found to be 4.1 degrees, which is smaller than for any alkane or alcohol system. Th e molecular size-dependent hysteresis suggests that contact angle hysteresi s phenomena are mainly due to liquid penetration and surface swelling, or a t least liquid retention, even on this very hydrophobic surface. The fact t hat receding contact angles reflect liquid retention by the solid and are t herefore not a property of the solid alone, supports the widespread practic e of using only the advancing contact angles in studies of surface energeti cs of solids and disregarding the receding contact angles. (C) 2001 Elsevie r Science B.V. All rights reserved.