Separation of oil from a water/oil mixed drop using two nonparallel plates

In this work, we have developed a simple approach to separate oil from a microliter-scaled water/oil mixture by squeezing the mixture using two nonparallel plates. Three pairs of plates with Teflon, SU-8, and SiO2 coatings, respectively, are used in the tests, and all of these plates are capable of separating the water/oil mixed drops. 95.5% silicone oil and 97.0% light mineral oil have been collected from their corresponding mixtures with water through the pair of Teflon plates. Furthermore, on the basis of pressure difference inside a liquid drop, theoretical models have been developed to interpret the corresponding mechanisms of the separation process, as well as the observed phenomena. To judge whether two immiscible liquids could be separated using the developed approach, a sufficient condition has also been derived, which includes three theoretical relations. The sufficient condition is subsequently validated by experiments. This condition also provides criteria for choosing a good plate coating. Such a coating should ensure (i) the oil wets the plate surface with a relatively large contact angle, and has small contact angle hysteresis, and (ii) the advancing contact angle that the water/oil interface forms on the plate surface is larger than 90°.