This article describes the design and manufacturing of a microfluidic chip, allowing for the actuation of a gas-liquid interface and of the neighboring fluid. A first way to control the interface motion is to apply a pressure difference across it. In this case, the efficiency of three different micro-geometries at anchoring the interface is compared. Also, the critical pressures needed to move the interface are measured and compared to theoretical result. A second way to control the interface motion is by ultrasonic excitation. When the excitation is weak, the interface exhibits traveling waves, which follow a dispersion equation. At stronger ultrasonic levels, standing waves appear on the interface, with frequencies that are half integer multiple of the excitation frequency. An associated microstreaming flow field observed in the vicinity of the interface is characterized. The meniscus and associated streaming flow have the potential to transport particles and mix reagents.
Actuators based on the controlled motion of a gas-liquid interface have applications in microelectromechanical systems (MEMS) [1][2][3][4][5][6][7][8], with the ability to move fluid or particles. The generation of bubbles is usually performed using either thermal [1] or electrolytic [2] methods. Both methods induce phase change, and the bubble grows within milliseconds. During the fast thermodynamic transformation, the surrounding pressure, temperature and electrical field can experience drastic changes, with consequences for the liquid close to the bubble, for instance in biomedical applications. In this study, an alternative is explored, where a gas-liquid meniscus is slowly injected from a microchannel into a microchamber, using a syringe pump. Since the bubbles in most MEMS applications are attached to at least one wall [2,4,5], a meniscus might replace a bubble advantageously. For instance, the control of the meniscus position, volume and interfacial pressure is much simpler than for a bubble, and menisci can be moved in microchannels using capillary forces [9]. Also, such system does not experience temperature pulses. Regarding the actuation of the gas-liquid interface, two types of processes are used in MEMS devices: (1) a large displacement (usually greater than 10 m) to directly push mechanical parts [3] or pump a liquid [5]; (2) high-frequency (on the order of 100 kHz [4]) oscillation of the interface with small amplitude to induce a steady microstreaming that mixes fluids or moves particles [7]. Using the first type of actuation process, Papavasiliou et al. [3] were able to displace a mechanical valve by about 100 m; Deshmukh et al. [5] could drive a micropump at 0.5 L/min (by periodic expansion and shrinking of a bubble, with the help of a passive microvalve), and Maxwell et al. [6] were able to capture and release particles by shrinking and expanding a microbubble in a cavity on the wall. Regarding the second type of actuation, Marmottant and Hilgenfeldt [7] observed a strong microstreaming field, in which particles can be transported. Also Kao et al. [4] showed that a micro-rotor can be driven at a speed as high as 625 rpm by the microstreamnig flow field induced by an oscillating bubble.
In this paper, we describe the manufacturing and characterization of microfluidic chip made of glass and cured Polydimethylsiloxane (PDMS). The potential of a gas-liquid meniscus to be used as an actuator in the chip is studied as follows. We first study the dynamics of large displacements of the meniscus (first type of actuation) by measuring the pressures needed to move the gas-liquid interface in a micro-geometry, and we classify the ability of different micro-features to anchor the interface at a desired location. Then, we study the response of the meniscus interface to high-frequency excitations. Besides a strong microstreaming flow in the vicinity of the interface, capillary traveling and standing waves are observed along the interface. We observe subharmonic capillary standing waves on the interface, which are analogous to Faraday waves [10]. Also, standing waves with superharmonic frequencies of the excitation frequency are observed. To the best of our knowledge, this is the first time that a superharmonic wave is found at a liquid-gas interface.
A typical microfluidic chip used in our study is shown in Figure 1: it involves a chamber (E) fed by a fork-like network of four channels A, B, C, D, with respective widths of 400, 1000, 400 and 100 micrometer. F is an interconnection needle and G is the clamp holding the chip. The height of each micro-channels is 50 µm. Channel D is used to inject air into the water-filled chamber E. The control of the gas-liquid interface position is enhanced by microgeometrical features at the junction of Channel D and the chamber, as shown in Figure 3 to 5. The microfluidic chip is manufactured using soft lithography [11] in the cleanroom of Columbia University, according to the following process. First, a 50µm thick SU-8 master (MicroChem) is cured on a silicon wafer with patterns transfered from a mask (CAD/Art Services Inc.). The chip is then manufactured from the master using PDMS Sylgard 184 Kit (Dow Corning). The PDMS chip is then covered with a PDMS cover plate and sandwiched between two glass slides hold by lateral clamps. This type of assembly ensures that all channel walls are made of PDMS and have the same surface properties. For some experiments involving high-frequency oscillations of the interface, a ring-shaped piezoelectric actuator is embedded in the PDMS cover plate, on top of the location shown by the letter B in Figure 1.
The experimental setup is described in Figure 2, and involves three subsystems: a microfluidic platform, a piezoelectric actuation system and a visualization system. The microfluidic system involves the microfluidic chip described above and allows for the controlled filling of the chamber and subsequent controlled injection of
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