Wetting of a glass substrate by a binary liquid mixture

Abstract

By reflecting light from a horizontal solid-liquid interface, we have measured the thickness of a gravity-thinned wetting layer in a stirred binary liquid as a function of temperature. The binary liquid studied is a mixture of polar and nondipolar liquids, nitromethane and carbon disulfide, respectively. At coexistence the lighter bulk nitromethane-rich ($N^{\mathrm{*}}$) phase floats above the heavier carbon-disulfide–rich ($C^{\mathrm{*}}$) phase. It is observed along the coexistence curve, while the liquid is stirred by a floating glass magnetic mixer, that a thick wetting layer of the $N^{\mathrm{*}}$ phase starts to develop on the borosilicate glass substrate at the bottom of the sample cell about 150 mK below the bulk critical temperature $T_c$. The wetting layer thickness, which is about 400 nm (the height spanned by the $C^{\mathrm{*}}$ phase is 0.82 cm), becomes constant through 7 K below $T_c$, while in the temperature range $T_c$-T$C^{\mathrm{*}}$ region, the $N^{\mathrm{*}}$ wetting layer increases in thickness as is expected for a retarded van der Waals interaction.