Cyclic Epipedography─Monitoring Contact Angle Changes in Dipping a Microsphere into a Liquid and Lifting from It

We propose cyclic epipedography to monitor the wetting characteristics of a microsphere on a stem with a liquid by tracking the level of a liquid line on the sphere when the sphere is dipped into and lifted from the liquid, and therefore the level of the liquid plane at infinity changes with respect to the sphere. Analysis of still images of a video taken with a horizontally held microscope determines the two levels. The sphere allows viewing of the liquid line without being obscured by the meniscus the liquid forms with its container's wall. A nearly perfect sphere is fabricated by melting the tip of a silica optical fiber while being rotated to form a ball 0.5-0.6 mm across. A dip-lift cycle is represented by a cyclic epipedogram─a closed loop in a plot of the level of the liquid line as a function of the level of the liquid plane. The position and shape of the loop allow us to estimate the contact angle at different stages in the dip-lift cycle. We applied the method to a hydroxylated silica microsphere in water, and the same sphere reacted with octylchlorosilane. Then, the method was applied to monitor the progress of the reaction of aminopropyldimethylethoxysilane with surface silanol. We find that the amphiphilic aminopropylsilane makes the surface conform to the environment. In air, the hydrophobic portion comes on top, while in water, hydrophilic part faces the surroundings. This conformity-caused Janus characteristics of the surface were almost absent with the hydroxylated silica and weak with the octylsilane-treated silica.