Understanding contact angle hysteresis on an ambient solid surface.
We report a systematic study of contact angle hysteresis (CAH) with direct measurement of the capillary force acting on a contact line formed on the surface of a long glass fiber intersecting a liquid-air interface. The glass fiber of diameter 1-2μm and length 100-200μm is glued onto the front end of a rectangular cantilever beam, which is used for atomic force microscopy. From the measured hysteresis loop of the capillary force for 28 different liquids with varying surface tensions and contact angles, we find a universal behavior of the unbalanced capillary force in the advancing and receding directions and the spring constant of a stretched meniscus by the glass fiber. Measurements of the capillary force and its fluctuations suggest that CAH on an ambient solid surface is caused primarily by two types of coexisting and spatially intertwined defects with opposite natures. The contact line is primarily pinned by the relatively nonwetting (repulsive) defects in the advancing direction and by the relatively wetting (attractive) defects in the receding direction. Based on the experimental observations, we propose a "composite model" of CAH and relevant scaling laws, which explain the basic features of the measured hysteresis force loops.
Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Yong Jian
YJ
Wang
Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Shuo
S
Guo
Department of Physics, National Central University, Jhongli 32001, Taiwan, Republic of China.|Institute of Physics, Academia Sinica, Taipei 11520, Taiwan, Republic of China.|Physics Division, National Center for Theoretical Sciences, Hsinchu 30113, Taiwan, Republic of China.
Hsuan-Yi
HY
Chen
Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Penger
P
Tong