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

Hydrodynamic cavitation (HC) occurs when a liquid’s static pressure drops to its saturation vapor pressure, generating, growing, and collapsing bubbles. While traditionally considered detrimental, HC has proven useful in numerous applications and is increasingly explored at the microscale, where phenomena differ from macroscale behavior due to surface nuclei, surface tension, and short residence times. Prior microfluidic cavitation-on-a-chip devices typically employed single micro-orifices or micro-Venturi elements, with visualization-capable Si-Pyrex or Pyrex-Si-Pyrex configurations, but generally operated under laminar conditions with limited pressure drops. Recent work introduced engineered sidewall roughness to facilitate nucleation and enable high-pressure operation, showing reduced cavitation inception thresholds and diverse flow regimes. This study aims to determine whether a new multi-orifice, eight-parallel-microchannel device can reduce the upstream pressure required for cavitation inception and enable more intense cavitating flows, and to assess how adding PVA microbubbles further facilitates inception and development. The purpose is to create a vigorous, multifunctional cavitation-on-a-chip platform suitable for microfluidic applications requiring tunable energy release (e.g., drug delivery, tissue engineering).

Early microscale cavitation studies by Mishra et al. introduced silicon–Pyrex devices with micro-orifices and micro-Venturi geometries, revealing distinct microscale behavior and strong scaling effects relative to macroscale cavitation (e.g., lower inception numbers, rapid transition to developed/supercavitating regimes). Medrano et al. demonstrated double anodic-bonded Pyrex–Si–Pyrex devices providing visualization while withstanding high pressures, with similar inception behavior to Si–Pyrex devices. Qiu et al. used controlled wet etching to create microsteps in Pyrex–Si channels, achieving HC at high flow rates without DRIE. Most prior devices handled laminar flows and modest pressure drops due to fabrication limits. Ghorbani et al. advanced the field with roughened sidewalls via DRIE “nanograss,” enabling high-pressure operation (up to ~6.2 MPa), and showed that engineered roughness reduces inception pressure and modifies flow patterns. Subsequent studies explored PVA microbubbles as cavitation facilitators, showing more facile cavitating flow generation and greater energy dissipation, and assessed roughness designs across many devices, finding short, small sidewall roughness most effective. Additional work with CNF-stabilized perfluorodroplets (PFC5) on nanoparticle-modified surfaces achieved supercavitation at low upstream pressures (~1.7 MPa), highlighting regenerative and energy storage potential. Prior designs largely used single restrictive elements; multi-element interactions were underexplored, motivating the present multi-channel cascade design to further reduce inception pressure and intensify cavitation.

Device design: A microfluidic device with eight parallel restrictive microchannels (arranged in a cascade) was designed. Flow enters a wide inlet chamber (2000 µm width allocated per nozzle) to damp transients before a sudden contraction into the nozzles to promote HC inception. Each nozzle has hydraulic diameter Dh = 66.6 µm and microchannel length L = 1 mm. Sidewalls incorporate engineered triangular roughness (length LR = 1/3 L; height HR = 0.1 Dh) to facilitate heterogeneous nucleation by modifying bubble tensile strength. Downstream, flow enters a high-pressure extension region (length 2 mm; width 7800 µm) to enhance bubble collapse, with outlets to the device sides. The design yields negative static pressure zones for bubble generation and a reverse pressure gradient for collapse, enabling a spectrum of cavitating regimes. Fabrication: Starting from double-side polished (100) Si wafers (525 µm), wafers were thinned to 250 µm and coated with 500 nm SiO2 on both sides. After HMDS priming, 4 µm AZ 9221 photoresist was spin-coated. Patterns were defined by maskless lithography (MLA150). SiO2 was etched by ICP (SPTS APS) at 10 °C (~3 min; 3:1 SiO2:PR selectivity), and PR was stripped. A second lithography step opened inlet/outlet and pressure port areas. DRIE (Alcatel AMS 200 SE) etched 200 µm Si at 30 °C (~80 min; 75:1 Si/PR selectivity), followed by PR stripping. To prevent wafer breakage during DRIE, 10 nm Ti and 2 µm Al were sputtered on the backside (Pfeiffer SPIDER 600) for protection and electrostatic clamping. Through-etching opened inlets, outlets, and pressure ports; completion was monitored by laser intensity. Protective layers were removed via wet etching: Al (phosphoric/acetic/nitric mixture, 35 °C), Ti (HF), and SiO2 (BHF). Piranha cleaning removed organics. Patterned Si wafers were anodically bonded to Borofloat-33 glass (Süss SB6) to seal microchannels. Experimental setup: A high-pressure N2 tank pressurized a steel liquid container feeding the microdevice via stainless steel tubing and fine valves within an aluminum package. Upstream and downstream pressures were measured with Omega sensors (±0.25% accuracy, up to 3000 psi). A glass cover enabled visualization using a Phantom VEO-710L high-speed CMOS camera (1280×800, pixel size 0.02 mm) with a 50 mm K2 DistaMax macro lens (f/1.2). Imaging was at 12,200 fps, 1000 s−1 shutter, 1 µs exposure, with front halogen backlighting; device-to-camera distance was 200 mm. Fluids and conditions: Two working fluids were tested: deionized water and a PVA microbubble (MB)–water suspension (1:19 mixture). PVA MBs were prepared per Cavalieri et al., and their concentration and size distribution were determined using a Neubauer counting chamber and transmitted light microscopy (16 images at 3000×4000 px analyzed in ImageJ). Upstream pressures from 0.2 to 1.1 MPa were applied. Cavitation inception and flow morphologies in the extension region downstream of the parallel channels were recorded and analyzed.

- The eight-parallel-channel cavitation-on-a-chip device enables cavitation inception at lower upstream pressures than prior single-restrictive-element devices, demonstrating the benefit of multi-element interactions and engineered sidewall roughness. - Adding PVA microbubbles further reduces the upstream pressure required for cavitation inception compared to pure water, facilitating earlier nucleation and faster intensification of cavitating flows. - The device can simultaneously exhibit multiple cavitating flow patterns of different intensities (heterogeneous nucleation, bubbly flow, partial cavitation with sheet and cloud structures, vortex formations) within the same extension region at a fixed upstream pressure and cavitation number, indicating multifunctional behavior and tunable energy release. - At an upstream pressure of 1.1 MPa, cavitation occurs in all eight parallel channels, with clearly visible partial cavities (sheet/cloud) and single-bubble events downstream. - Cavitating flows intensify faster in this multi-channel device for both water and PVA MB–water compared to previous single-channel designs. - Geometric and operating specifics: nozzle Dh = 66.6 µm; microchannel length L = 1 mm; roughness LR = 1/3 L and HR = 0.1 Dh; extension region 2 mm × 7800 µm; tested pressure range 0.2–1.1 MPa.

The findings confirm that configuring multiple parallel restrictive microchannels with engineered sidewall roughness decreases the input energy required for hydrodynamic cavitation inception and accelerates the transition to developed cavitation. The multi-element configuration likely promotes local pressure minima and interaction-driven instabilities that favor nucleation and sustained cavitation across channels. Incorporation of PVA microbubbles provides additional nucleation sites, reducing the inception threshold further and enhancing cavitation dynamics, consistent with prior observations of MB-facilitated cavitation. The ability to generate diverse cavitation morphologies concurrently within a single device at a fixed pressure/cavitation number highlights the platform’s potential as a multifunctional microreactor, enabling spatially varying energy release for applications such as drug delivery, tissue engineering, and microscale processing. Overall, the device addresses the research aim by demonstrating lower inception pressures, faster intensification, and rich flow regimes compared to earlier single-element designs, advancing cavitation control at the microscale.

A next-generation cavitation-on-a-chip device with eight parallel, roughness-engineered microchannels was designed, fabricated, and experimentally validated. The device achieves cavitation inception at reduced upstream pressures relative to single-element predecessors and exhibits rapid intensification and simultaneous multiple cavitation regimes. Using PVA microbubbles further lowers inception pressure and enhances cavitation development. These capabilities position the device as a promising, energy-efficient platform for microfluidic and organ-on-a-chip applications requiring controlled, tunable cavitation-driven effects (e.g., targeted drug release, tissue engineering, microscale material processing). Future work could quantify inception pressure reductions versus specific single-channel baselines across broader geometries, optimize roughness parameters and channel interactions, map spatiotemporal cavitation energy release, and integrate on-chip sensing/actuation for closed-loop control.