# Plasma Surface Modification of PDMS-Glass Microfluidic Chips for Oil Recovery Studies
## 3 Results and Discussions
### Modification of Chip Capillaries  
As noted above, PDMS, from which the chips were made, is a porous hydrophobic material. After plasma treatment, polar groups are formed on its surface, which have a short lifetime, since they are chemically very active and can react with crude oil components. To stabilize the surface, we used salt water, which forms Helmholtz adsorption layers and thus can protect the modified surface while maintaining its hydrophilicity. In addition, we assume that the water fills the pores of the PDMS and makes them inaccessible to oil wetting. To test the performance of the technology, two chips with the same topology were made from PDMS and glass, imitating the porous structure of rocks, and they were hydrophilized according to the method described in the previous section. Both chips were filled with oil and this oil was displaced by water, thus simulating the process of development by waterflooding of oil-saturated rocks with conditionally hydrophobic and hydrophilic properties. Observations of the displacement process (Table 1) showed that the W chip after washing contained less oil than the O chip. At the same time, in the first minutes of washing, many thin, fast water jets formed at the beginning of the W chip chamber, which later merged into one wider one. Over time, the channel of the water jet expanded due to the displacement of oil from the far part of the chip, but many oil \"islands\" at the beginning of the chip did not move. In chip O, a different displacement pattern was observed: the front of the water flow was even, and the intensive expansion of the resulting water channel occurred in the part of the chip closer to the inlet channel; fewer oil \"islands\" were formed, and they were smaller. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Time, min** & **Chip Primary Extracted in oil (O)*** & **Chip Primary Extracted in Water after Plasma Treatment (W)*** \\ \hline \hline
1 & **1** & **2** \\ \hline \hline \end{tabular}  
* In all photos, the pumping occurs from left to right.  
\end{table} 
Table 1: Patterns of oil displacement in chips depending on the duration of this displacement. After processing the obtained data, the dynamics of the oil displacement efficiency for both chips were obtained (Figure 9). It can be seen that the displacement of oil occurs most actively in the first minutes of the experiment. In chip W, the initial water jet broke through the oil layer faster (compared with chip O) and had a smaller width, thus forming a channel in a shorter period of time. This resulted in a comparatively lower \(K_{dis}\) in the first 5 min of displacement. However, the boundaries of the formed channel in the W chip further expanded faster, which led to an increase in \(K_{dis}\). Thus, it was shown that plasma treatment of a PDMS microfluidic chip followed by primary aqueous extraction leads to an increase in the oil displacement efficiency from the chip by about 10%. We explain this by the fact that plasma treatment makes the nanopores of PDMS water-wettable, and thus available for filling with water. Because of this, hydrophilization of the surface, which is resistant to crude oil, occurs. To obtain quantitative characteristics of the effect of PDMS plasma treatment on the volume and depth of water absorption into the surface, an additional study of the surface topographic characteristics, including the analysis of XPS data, is necessary. After the oil was displaced by water, the chips were examined under a microscope (Figure 10). The meniscus of the oil remaining between the columns shows that the wetting pattern in the W chip is significantly different. Although, as can be seen in general, the surface remains hydrophobic. In the O chip, oil slicks are visible in the water channels adhering to the lower and upper walls of the chip. In the W chip, such spots are either not observed, or they had a less pronounced color, which indicates a significantly smaller thickness of this drop compared with those observed in the O chip.