The effect of liquid properties to contact angle hysteresis

Low-rate dynamic advancing and receding contact angles of 21 liquids from two homologous series (i.e. n-alkanes and 1-alcohols) and octamethylcyclotetrasiloxane (OMCTS) on FC-732-coated silicon wafer surfaces were measured by an automated Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The receding contact angle was found to decrease with time, suggesting surface swelling and sorption. An initial receding angle (θri), which theoretically represents the receding angle before sorption starts, was extrapolated back to time of zero contact (t0) by least-squares regression in each experiment. The contact angle hysteresis, i.e. the difference between advancing contact angle and θri, was found to decrease with increasing chain length of the liquid molecules for both the alkane and alcohol series. Contact angle hysteresis was found to vanish when the chain length was extrapolated to infinity. These results are plausible in terms of liquid sorption by the solid surface: very large molecules are unlikely to penetrate into the solid film. In order to reconfirm that contact angle hysteresis depends on molecular size of the liquid, contact angles of OMCTS, which is a bulky and quasi-spherical molecule, were measured. The hysteresis was found to be 4.1°, which is smaller than for any alkane or alcohol system. The molecular size-dependent hysteresis suggests that contact angle hysteresis phenomena are mainly due to liquid penetration and surface swelling, or at least liquid retention, even on this very hydrophobic surface. The fact that receding contact angles reflect liquid retention by the solid and are therefore not a property of the solid alone, supports the widespread practice of using only the advancing contact angles in studies of surface energetics of solids and disregarding the receding contact angles.