# Plasma Surface Modification of PDMS-Glass Microfluidic Chips for Oil Recovery Studies
## 4 Conclusions  
A simple and accessible technology for modifying the surface of PDMS and glass using a dielectric barrier discharge in an air atmosphere saturated with water vapor at a temperature of 25 \({}^{\circ}\)C and a pressure of 5 kPa has been developed. The wettability characteristics (advance and recede angles) are measured experimentally as a function of plasma treatment time for one of the most common materials used in microfluidic technologies: PDMS and glass. To test the efficiency of the technology, microfluidic chips were made to study oil recovery and hydrophilization of their surface was carried out. The effect of hydrophilization of the surface of microfluidic chips on the pattern of two-phase flow in them during oil displacement has been studied. As a result, the following conclusions were obtained:  
* It is shown that with the help of plasma treatment it is possible to change the wetting angles of the walls of microfluidic chips in a very wide range, thereby simulating the conditions of both hydrophobic and hydrophilic rocks;
* PDMS has a very wide contact angle range from hydrophobic to super-hydrophilic: the contact angle decreases from 120\({}^{\circ}\) to 10\({}^{\circ}\); receding angle--from 70\({}^{\circ}\) to 0\({}^{\circ}\) during plasma treatment;
* Glass in its original state is a hydrophilic material. It is shown that even short-term plasma treatment makes it possible to achieve superhydrophilicity of the glass surface. For 1 min of treatment, the contact angle decreases by more than 50\({}^{\circ}\). This material is recommended for use at high pressures and relatively high temperatures due to its high strength, chemical resistance and the possibility of plasma-chemical surface modification;
* The study of the processes of oil displacement from hydrophilized chips showed that the patterns of water-oil flow in the treated and untreated chip differ significantly on the macro- and microscales. Despite the fact that the overall surface of the chips remains hydrophobic. It was shown that plasma treatment of a PDMS microfluidic chip leads to a significant change in the wetting characteristics of its surface. This in total leads to an increase in the oil displacement efficiency from the chip by about 10%. In general, the results of the study showed that plasma surface treatment for the hydrophilization of microfluidic chips is a simple and affordable technology for controlling the wetting characteristics of microfluidic chips. This is a very important aspect, since the inability to obtain wetting characteristics close to real rocks is one of the significant issues that are often presented to microfluidic technologies for studying oil recovery. The  
Figure 10: Microscopic photographs of the area where oil has been displaced in the chips primary extracted in oil (**a**) and water after treatment in the plasma chamber (**b**). presented method of surface modification makes it possible to partially remove these limitations. Additionally, for these purposes, PDMS is one of the most promising materials for modeling rocks, since it is porous at the nanoscale and, similar to a natural rock, its wettability depends on the primary extraction. Conceptualization, A.S.Y. ; methodology, M.I.P. and A.S.Y. ; validation, M.I.P., A.V.M., A.I.P. and A.S.Y. ; formal analysis, A.V.M., A.S.Y. and A.D.S. ; investigation, A.I.P. and A.S.Y. ; resources, A.S.Y. and A.V.M. ; data curation, A.I.P. ; writing--original draft preparation, A.S.Y. ; writing--review and editing, A.V.M., A.I.P., M.I.P. and A.D.S. ; visualization, A.S.Y. and A.I.P. ; supervision, A.V.M. ; funding acquisition, A.V.M. All authors have read and agreed to the published version of the manuscript. This research was funded by Ministry of Science and Higher Education of the Russian Federation (State assignment of the Siberian Federal University FSRZ-2020-0012). Not applicable. Data Availability Statement: Data is contained within the article.