Design and fabrication of a vigorous “cavitation-on-a-chip” device with a multiple microchannel configuration

Hydrodynamic cavitation (HC), a phase change phenomenon initiated by a pressure drop below the liquid's saturation vapor pressure, involves bubble generation, growth, and implosion. While often undesirable, HC finds applications in various fields. Microscale HC presents unique challenges and opportunities due to scaling effects, the influence of surface tension, and the residence time of nuclei. Previous research on microscale cavitation devices focused on single micro-orifice or Venturi designs, often limited to laminar flow and low-pressure drops. Recent advancements incorporated surface roughness to enhance cavitation inception, but these were still single-channel systems. This study aims to create a multi-channel device to further reduce the pressure required for cavitation inception and generate more intense cavitation using multiple flow restrictive elements.

Early work by Mishra et al. explored microscale cavitation using micro-orifice devices, highlighting scaling effects. Subsequent studies investigated micro-Venturi geometries and the impact of geometry on cavitation generation. These studies emphasized the differences between macro and microscale cavitation, with surface nuclei playing a more significant role at the microscale. Medrano et al. addressed challenges in visualization by fabricating double anodic-bounded Pyrex-silicon-Pyrex devices. Qiu et al. presented a simplified fabrication method to generate HC in microchannels. Ghorbani et al. introduced devices with roughened sidewalls to improve cavitation inception under turbulent conditions, investigating the effects of surface roughness. They also explored the use of PVA microbubbles (MBs) to facilitate cavitation and assessed the potential of using cellulose nanofiber-stabilized perfluorodroplets. These studies laid the groundwork for the current research, focusing on enhancing cavitation inception using multiple microchannels.

The new device consists of an inlet channel, an inlet chamber, eight parallel microchannels (66.6 µm hydraulic diameter), and an extension region. The microchannels have sidewall roughness elements (triangular, with parameters LR and HR). The device was fabricated using double-side polished silicon wafers, SiO2 coating, photolithography, inductively coupled plasma etching (ICP), deep reactive ion etching (DRIE), titanium and aluminum sputtering, and anodic bonding with Borofloat-33 glass. Experiments used water and a PVA MB/water suspension (1:19 ratio) as working fluids, with inlet pressures ranging from 0.2 to 1.1 MPa. A high-speed camera recorded cavitation flow patterns. PVA MB concentration and size distribution were determined using a Neubauer counting chamber and ImageJ software.

The multi-channel device successfully generated cavitation bubbles in all eight parallel channels at 1.1 MPa. Different cavitation flow patterns (bubbly flow, sheet cavitation, cloud cavitation, vortex formation) were observed simultaneously within the device at the same upstream pressure. The use of PVA MBs further facilitated cavitation inception at lower pressures. The device demonstrated a significant reduction in the upstream pressure required for cavitation inception compared to previous single-channel devices. The intensity of cavitating flows increased more rapidly in this device compared to previous designs, suggesting more efficient energy utilization for cavitation generation. Figure 4 showed cavitation bubble generation inside the device with eight parallel micro-channels. Figure 5 shows cavitating flows at 1.1 MPa in the extension region downstream of two parallel channels (channels 4 and 5).

The results demonstrate the effectiveness of the multi-channel design in facilitating cavitation at lower upstream pressures. The parallel arrangement of microchannels enhances cavitation inception by creating multiple nucleation sites and promoting interactions between the flow restrictive elements. The observed variety of cavitation patterns suggests the device's potential for applications requiring different levels of energy release. The use of PVA MBs further optimizes the process, reducing the energy input needed for effective cavitation. These findings provide a significant advancement in the field of microscale cavitation devices, paving the way for more energy-efficient and versatile applications.

This study successfully demonstrated a novel multi-channel ‘cavitation-on-a-chip’ device capable of generating intense cavitation at significantly lower upstream pressures than previously reported. The parallel microchannel design, combined with surface roughness and the use of PVA MBs, offers a highly efficient and versatile platform for various microscale applications. Future research could explore optimizing the device geometry for specific applications and investigating the effects of different MB types and concentrations.

The current study focused on a specific device geometry and a limited range of working fluids. Further investigation is needed to assess the device's performance with other fluids and different geometrical parameters. The impact of long-term operation on device performance and the potential for fouling also warrant future investigation.