Static Friction of Agar Gels: Formation of Contact Junctions at Frictional Interface

We measure the static friction F c of agar gels on glass substrates immersed in water. F c is independent of the nominal contact area, and increases with the normal load and the duration t w of contact prior to sliding. Using a confocal laser-scanning microscope, many fine dark spots are clearly visible in the optical reflection distribution images on the glass interface in contact with the gel. The total area of the dark spots increases with t w , corresponding qualitatively to that of F c . These observations indicate that F c originates from the formation of the contacting spots at the interface, which is not due to asperities of the gel surface alone but related to the drainage of water trapped by the gel polymer network at the frictional interface.