ABSTRACT

If a small liquid drop is deposited on a surface, it spreads. If additional liquid is added to the drop, the contact line advances and stops. Each time motion ceases, the drop exhibits an advancing contact angle 0a, as shown in Fig. 1. Alternatively, if liquid is removed from the drop, the contact angle decreases to a receding value 0r, then the contact line recedes. Contact angle hysteresis (0a - 0r) has been studied widely, but the fundamental underlying causes are not completely understood. It often is attributed to large-scale surface heterogeneities or roughness [1]. How­ ever, it may not be necessary to invoke surface imperfections to explain hystere­ sis. It has been suggested that physicochemical interactions between the liquid and solid alone may be sufficient [2-7].