2.2.1. Hydrophobic/Hydrophilic
Since and are constant, it is known from the Young’s equation that the contact angle decreases accordingly when the surface tension between solid and liquid decreases, leading to the increase of adhesion energy standing for hydrophilic characteristics. Additionally, vice versa, the contact angle increases standing for the hydrophobic characteristics. A surface with droplet water contact angle greater than 90° is hydrophobic, but less than 90° is deemed hydrophilic. The affinity between a material and water is described by the term hydrophilic/hydrophobic. Materials with polar groups usually have great affinity for water and can attract water molecules to be easily wetted. On the contrary, hydrophobic materials tend to be non-polar with no affinity for water and are not easily wetted. For hydrophobic materials, when water comes into contact with the surface of the material, the contact angle is generally greater than 90°. Droplets that are large enough to experience gravitational forces that exceed the capillary forces are likely to break up into small droplets or beads. The opposite is true for hydrophilic materials, where small droplets will aggregate into a film. As the water molecule is polar, materials with polarizable surface groups tend to be hydrophilic, having contact angles that are less than 90°.
Common hydrophilic substances are aluminum, zinc, and other metals and their oxides, glass and mica, quartz, talc, calcite, quartz, and many other minerals. In addition, the single and associating -OH polar groups on the surface of the material, which can form hydrogen bonds with water molecules, are hydrophilic. The hydrophobic group is mainly represented by -NO
2, Si-H, and Si-CHx groups, and the Si-F group also exists in a small amount [
57], such as paraffin, Teflon (PTFE), polyamide (PA), PC (polycarbonate), PAN (polyacrylonitrile), fluorinated polyethylene, fluorocarbon wax, polyolefin, polyester, fluoro-free acrylates, and so on. Modifying the surface of the material to chang the hydrophilic and hydrophobic nature can change the contact angle, which is an important method to construct the surface tension valve.
Due to the capillarity property generated by surface tension, water is pulled into a microchannel with a hydrophilic surface inside, but it meets the stop barrier at the hydrophobic surfaces. Taking advantage of hydrophobic–hydrophilic interface (hydrophobic—more hydrophobic; hydrophilic—more hydrophilic) effect can be used to manage the flow of liquid in the microchannel [
58].
2.2.2. Material Properties
Material selection is the first step in the fabrication of microfluidic chips, which directly affects the function of chips and determines the processing and production methods of subsequent chips. Chips made of different materials have different production costs, processing difficulty and specific processing methods. The selection of chip materials is also related to the observation and detection of subsequent experiments. Different chip materials have a direct impact on the difficulty of optical detection in subsequent experiments due to their different light transmittance. CPCV is an important part of a chip, and different properties of different materials also directly affect the design and production of CPCV.
The materials for making microfluidic chips generally include silicon, glass and a variety of polymer materials, for example, polymethylmethacrylate (PMMA), polydimethylsiloxane (PDMS), polystyrene (PS), and Polycarbonate (PC), etc.
Table 1 shows the advantages and weakness of common materials used to fabricate microfluidic chips, where the contact angles with water were summarized.
It can be seen from
Table 1 that, compared with silicon and glass, low cost and easy processing are advantages of polymer materials, so polymer materials are mostly used in the production of microfluidic chips. PDMS is a widely used polymer material for microfluidic chip processing. PDMS is chemically inert and non-toxic; it has excellent light permeability and is convenient for optical detection; with good plasticity, it is easy to process; it is convenient for surface modification and bonding; it can be repeated many times for mass production. In addition, most of the polymer materials are naturally hydrophobic, such as PDMS (contact angle: 113.5 ± 2° [
59]) and PMMA (contact angle: 97° [
60]), and the surface performance is not stable. After hydrophilic treatment, there is the phenomenon of hydrophobic recovery, which will gradually lose hydrophilicity and return to hydrophobic state. This is very unfavorable for CPCV constructed by changing surface hydrophilicity and hydrophobicity, and seriously affects the application expansion of CPCV.
Table 1.
Properties of different materials for preparing microfluidic chips.
Table 1.
Properties of different materials for preparing microfluidic chips.
Materials | Advantages | Weaknesses | Contact Angles with Water (°) |
---|
Silicon | Chemical inertness Smooth surface Mature technology Easy to mass production | Fragile High cost Opaque Complex surface chemistry | Silica (super-hydrophobic): 161 [61] Silica (conventional): 24.5 [61] Silicon (FDTS): 113.7 ± 3.1 [62] Silicon (DDMS):105 [63] |
Glass | Good electroosmotic Good optical properties Easy for surface treatment | Fragile, high cost Bonding difficult Difficulty in a large aspect ratio | Glass (uncoated): 68.5 [64] Glass (2% APTES): 40 [65] Glass (MSNPs-CVD): 175 ± 2 [66] Glass (MSNPs-Sol): 158 ± 2 [66] |
Polymer | Variety and low cost Transmission of light Easy to process and form Cheap mass production | Low heat-resistant Low thermal conductivity | PDMS: 113.5 ± 2 [59] PMMA: 97 [60] PDMS (APTES+MA): 60 [67] |
2.2.3. Surface Treatment Methods
The most commonly used materials for microfluidic devices, and therefore for surface tension valves, are glass and PDMS. The chemical stability of PDMS is very good, so its surface modification is quite difficult. In addition, the surface properties of PDMS are unstable. Even if the surface of PDMS becomes hydrophilic through modification, the surface will gradually lose hydrophilicity and return to the hydrophobic state, a phenomenon known as hydrophobic recovery [
68,
69]. The mechanism of PDMS surface instability is not yet clear. Most surface modification methods have not solved the problem of PDMS hydrophobic recovery. Common surface treatment methods include plasma treatment [
70,
71], covalent surfactants treatment [
72], and Sol–gel coating [
73,
74].
Plasma treatment: this surface treatment method is currently the most commonly used method for surface modification of PDMS [
69]. Plasma is an ionized gaseous substance composed of atoms deprived of some electrons and positive and negative ions generated after the ionization of atomic groups. It is often regarded as the fourth state of matter in addition to solid, liquid, and gas. In plasma treatment, oxygen plasma is used to react with the surface of the material to expose its hydrophilic chemical functional groups. When a surface is treated with hydrophilic plasma, it increases the surface energy of the object. When the surface energy of an object is high, its adhesion is high. However, plasma surface modification of PDMS can restore its hydrophobic properties within a few minutes, as exposed hydrophilic groups recombine with uncured hydrophobic polymer chains, causing the surface to lose hydrophilic properties. Maintaining the surface of the material in water immediately after plasma treatment or using solvent extraction to remove the uncured polymer can effectively slow down or prevent this recovery [
70,
71].
Surfactants treatment: surfactant molecules have charged “heads” and hydrophobic “tails” that can easily adsorb onto the hydrophobic surface and change its surface properties. Amphiphilic surfactant molecules enter the microchannel by running the buffer, the hydrophobic tail is physically adsorbed on the PDMS surface, and the hydrophilic surfactant head is extended into the buffer, making the PDMS surface hydrophilic in situ, to achieve the purpose of changing the surface properties of PDMS. This method not only reduces the cost of surface modification, but also makes it more rapid and simple. As the surfactant does not form a strong covalent bond with primary PDMS only by weak hydrogen bond binding, desorption can occur, and excessive surfactants are needed to dynamically supplement the desorption substances in the process, so as not to have a negative impact on the surface performance.
Nowadays, a large number of surfactants are applied to PDMS surface modification. For hydrophilic treatment, there is poly ethylene glycol methacrylate (PEGMA), 2-hydroxy ethyl methacrylate (HEMA), O2 plasma, tween-20 [
75], 3-aminopropyl triethoxysilane (APTES) [
67,
76], polyvinyl alcohol (PVA) [
72], polyvinylpyrrolidone (PVP) [
77], Pluronic [
78], etc. For hydrophobic treatment, there is octadecyltrichlorosilane (OTS) [
79], Sigmacoat [
80], Fluorosilane coupling agent, Saline, etc.
Sol–gel coating: in this method, the “solution” and “sol” of compounds containing high chemical active components are cured into a “gel” state through a series of treatments. Sol–gel technology can rapidly and repeatedly construct a large number of ordered hydrophilic and hydrophobic surface structures on nanoscale surface, which is a promising surface modification method at present [
81,
82,
83,
84,
85]. The Sol–gel coating can be produced by electrophoretic deposition, impregnation, and sputtering. At present, the deposition coating mainly uses silica sol, which is composed of alkoxy compound and its composite material with metal salt solution. Hydrophobic coatings can be formed directly by introducing a hydrophobic agent (hexamethyl-disilazane, trimethylchlorosilane) into the aerosol and curing it on the material surface. In recent years, the preparation of hydrophobic and super-hydrophobic surfaces has received extensive attention, among which the use of organosilicon fluoro compounds, especially containing hydrolytic groups, is the key development direction [
86,
87,
88]. Fluorinated compounds can be used not only as modification additives in the preparation of Sol–gel composites used for coating formation, but also as the main components of hydrophobic coatings [
89].
According to different materials and application scenarios, a large number of surface modification methods have been proposed. Surface modification is a popular research field, especially super-hydrophobic surfaces (which provide a wet angle of more than 150 degrees), including Nano-surface [
86,
87,
90], which has been a new research hotspot in the past two decades [
74,
89,
91]. The surface modification method of PDMS mentioned above is also applicable to common microfluidic materials such as plexiglass. For example, hydrophobic glass can be obtained by preparing the surface of glass by Sol–gel method [
92]. There are also other methods, such as ultraviolet (UV) treatment [
93,
94], chemical vapor deposition (CVD) [
95,
96], self-assembled monolayers (SAMs) coatings, etc.
By changing the contact angle of the material through any of these methods, the Laplace pressure of the liquid interface is changed. This shift in Laplace pressure is essential for the manufacture and design of the CPCV.