# Plasma Surface Modification of PDMS-Glass Microfluidic Chips for Oil Recovery Studies
## 1 Introduction  
Capillary fluid flows are widely used and studied in microfluidic devices, which are structures with one or more capillaries that perform certain manipulations with the flow of liquids or gases in them. For example, in microfluidic devices, one can obtain an emulsion [1], separate cells in a stream according to certain characteristics [2], conduct enzyme immunoassay [3], and much more [4]. The most popular manufacturing technology for microfluidic devices is lab-on-a-chip. Microfluidic devices made using this technology are referred to as \"microfluidic chips\". A distinctive feature of microfluidic chips is that the capillary network is located in one plane--at the junction of two hermetically connected plates, on one of which (sometimes on both) a deep channeled pattern is formed, called the microfluidic chip topology. The walls of this pattern are the walls of the capillary network. Most often, transparent materials are used for the manufacture of microfluidic chips, since they allow visual control of processes in flows, as well as quantitative measurements by optical methods. Microfluidic chips are made of glass [5], thermoplastic polymers [6], and elastomers [7]. The channeled pattern is formed by soft lithography [8], laser ablation [9], mechanicalengraving [10], and taking an impression from a previously made master mold [11; 12; 13; 14]. In addition to the formation of a channeled pattern, an important stage in the manufacture of microfluidic chips is their sealing, during which the topology of the chip is sealed and a capillary network is formed. Sealing is carried out by gluing with adhesives [15], solvents [16], thermal sintering [17], and chemical crosslinking [18; 19; 20; 21]. Often, before sealing, the surfaces to be joined must be activated using plasma or ultraviolet light [22; 23; 24]. When sealing, a change in the geometric characteristics of the chip topology can occur. Thus, the choice of material is due to the presence of both technologies for it: micromachining and sealing. In addition to the geometric properties of the capillary network of chips, the chemical and physical characteristics of the surface of the capillaries are of key importance: the chemical composition of the surface and wettability. For many microfluidic applications, the natural surface properties of the materials from which microfluidic chips are made are not suitable. For example, certain biochemical groups must be attached to the surface of enzyme immunoassay test strips to ensure the specificity of analyte binding in order to increase its local concentration and the possibility of visual detection [25]. Another example where the physical characteristic of wettability is important is the simulation in porous chips of multiphase flows that are observed in rocks (for example, when studying oil recovery during reservoir flooding). In such chips, it is necessary to accurately recreate the wetting angle of the capillary surface similar to that observed on natural rock grains [26]. It is known that in nature the wettability of natural rocks dominated by silicates is primarily determined by primary extraction [27]: if most nano- and micropores are filled with water, then the surface will be hydrophilic; if the pores are filled with non-polar liquids, then such a surface will be hydrophobic. Polydimethylsiloxane (PDMS), from which chips are made for modeling flows in rocks [28], is a porous material with a contact angle of about 120\({}^{\circ}\). With this initial contact angle, the primary water extraction does not lead to an increase in hydrophilicity, since water does not enter the pores of the PDMS. It is clear that the wettability of a surface depends on its chemical composition. Therefore, to change the wettability, the surface of the capillary walls undergoes modification, during which the chemical composition of the thin surface layer changes.