Contact angle of ethanol-water solutions on crystalline and mesoporous silicon

Measurements of contact angle of ethanol-water solutions were performed on crystalline silicon and on mesoporous silicon films with porosities up to similar to 72%. Water contact angles of 44 degrees and 76 degrees were measured for untreated and HF-dipped crystalline silicon, respectively, consistent with previous studies. The contact angle for ethanol-water mixtures was found to decrease with increasing ethanol concentration for both untreated crystalline silicon and also for HF-dipped crystalline silicon up to an ethanol concentration of similar to 80%; at higher concentrations the contact angle approached zero. Similar behaviour was observed on mesoporous silicon surfaces for ethanol concentrations less than or similar to 40%, above which the contact angle dropped abruptly to an immeasurably small value. This behaviour is attributed to a decrease in surface tension with increasing ethanol concentration. For all ethanol-water solutions, a minimum value of contact angle was observed at a porosity of less than or similar to 40%, above which it remained approximately constant. The behaviour of contact angle with porosity can be explained by a change in the Wenzel roughness parameter due to changes in the specific surface area of the film.