Kinetics on formation of super water repellent surfaces from phase transformation in binary mixtures of trimyristin and tripalmitin

Triglyceride surfaces were prepared by solidification from the melting of binary mixtures of trimyristin (MMM) and tripalmitin (PPP) with different mole fraction (xPPP) to further investigate the formation mechanism of such super water-repellent surfaces. The hydrophobicity change of the MMM/PPP surfaces was monitored by the measurements on time-dependent contact angle. It has been found that all the mixture samples can form super water-repellent surfaces spontaneously under appropriate heat treatment conditions. However, the time (t150) needed to exhibit super water-repellency for an MMM/PPP surface, incubated at a specific temperature, decreases slightly along with xPPP at the range from 0 to 0.2, while the t150 value increases steadily at the xPPP range from 0.2 to 1.0. The plot of t150 versus xPPP has the similar trend to the solid-liquid phase diagram with a eutectic point of the binary system determined by differential scanning calorimetry (DSC). The fractal dimensions (FDs) describing roughness of these surfaces were calculated by the box-counting method from the scanning electron microscopy (SEM) images of different water-repellent surfaces and their cross sections. Kinetic constants of the formation of super water-repellent surfaces are further obtained and quantitatively compared by the correlation on the basis of the time-dependent contact angles.

(a) Time-dependent contact angles and some photographs of water droplets on MMM/PPP mixture surfaces with the composition of xPPP = 0.4 incubated at different temperatures. (b) SEM images and cross-sectional trace curves of MMM/PPP mixture surfaces (xPPP = 0.6) incubated at 40 °C for 150 min with the contact angle of 153.5°.Figure optionsDownload as PowerPoint slideHighlights
► Binary mixtures of trimyristin and tripalmitin (MMM/PPP) are used in this work.
► All binary mixtures can form super water-repellent surfaces spontaneously.
► Phase diagram of the binary mixtures is determined by DSC.
► Kinetic constants for the formation of super water-repellent surfaces are obtained.