Design and Characterization of Microfluidic Devices for Water/Oil Separation

Demulsification is the breaking of immiscible fluids into two phases, i.e. removing water in oil or oil in water. It is a fundamental step in oil processing. The main difficulty of demulsification is destabilizing emulsions to a satisfactory level for successive steps. Demulsification techniques can be grouped into three main categories: chemical, biological, and physical treatments. They are often combined to improve destabilization of the emulsions. In this paper we propose a preliminary study for preparing suitable oil samples for analytical investigation, based on a microfluidic approach for water separation from water in oil (WO) emulsions. We also suggest the possibility of using scanning micro-Fourier Transform Infrared Spectroscopy (μ-FTIR) to analyze oil samples containing drops of water of micro-dimensions. A glass/silicon microfluidic device characterized by a snake-like microchannel (200 μm wide and 100 μm high) ending in a Y-junction is proposed. The total volume of the microfluidic device is 3 μl. WO separation is favored by an asymmetric chemical functionalization of the inner surfaces of the microchannel. The surface functionalization process is indeed performed by a wet-chemistry process to make half of the channel hydrophilic, while turning the opposite surface hydrophobic. Taking advantage of the laminar flow spontaneously established inside the microchannel, this approach favors the spontaneous separation of water from oil along the microchannel itself while flowing. Water drops firstly appear at the sidewall of the channels and then they coalesce to form larger drops. Separation of water from oil is obtained at the Y-junction, which is designed to have one branch totally hydrophobic and the other one completely hydrophilic. At the two ends of the microfluidic device two samples can be collected, just oil at the hydrophobic side and a water in oil system at the hydrophobic side, with large water drops. Scanning micro-Fourier Transform Infrared Spectroscopy (μ-FTIR), both in reflectance and transmission modes, was used to analyze WO emulsions. We demonstrated that this analytical technique can be successfully used to characterize WO emulsions if water drop dimensions are in the micrometer range, obtaining signals with no significant noise or disturbance related to the presence of water.