A robot-assisted acoustofluidic end effector

Many chemistry, biology, pharmaceutical, diagnostic, and clinical workflows rely on repetitive microscale liquid handling steps that are time-consuming, error-prone, and expensive in reagents and skilled labor. Existing macro-scale robotic liquid handlers largely automate pipetting and mimic human operators but consume large reagent volumes and lack microscale preparation capabilities. In parallel, microfluidics and lab-on-a-chip technologies achieve precise microscale manipulation but generally lack automation and multifunctionality. An effective interface between macro-level robotics and micro-level microfluidics is missing, limiting integrated, automated microscale workflows. Ultrasound-based micromanipulation can trap and manipulate particles and fluids at small scales, but typical devices operate in limited environments, require manual frequency tuning for dynamic manipulation, and face challenges in 3D control and selective trapping. This work aims to address these gaps by integrating acoustofluidics as a multifunctional end effector on a robotic arm to enable automated, flexible, and programmable microscale liquid and particle manipulation.

Prior end effector strategies include electrostatic, magnetic, and pneumatically controlled soft grippers primarily suited for millimeter–centimeter objects, with limited success manipulating microscale objects using robotic arms. Acoustic approaches have trapped microparticles using standing bulk and surface acoustic waves, Chladni plates, oscillating sharp edges, ciliary arrays, and microbubbles, but these typically operate in constrained environments and lack the spatial flexibility of robotic arms. Dynamic acoustic manipulation often requires tuning resonance frequency within limited transducer bandwidths, restricting range; 3D manipulation and selective trapping remain difficult. Recent progress using micromanipulators includes spiral-electrode acoustic tweezers for single-cell handling and a whirling-flow device using a piezo-actuated micropipette, but broad robotic integration for functions such as pumping, trapping, and mixing has not been demonstrated. Limited robotics–microfluidics integration exists via digital microfluidics, modified inkjet heads, and robot-assisted electroporation; a robotic cap-to-dispense device showed pipette-free droplet dispensing. There remains a need for a versatile, programmable interface marrying robotics’ motion planning and automation with microfluidics’ precision control.

System architecture: The RAEE consists of a five-degree-of-freedom robotic arm carrying an acoustofluidic end effector comprising a hollow borosilicate glass capillary bonded to a piezoelectric transducer. The capillary has ~1.5 mm outer diameter at the base, tapering to ~3–10 μm at the tip. The transducer (7.0 × 8.0 × 0.2 mm, resonance ~240 kHz) is driven by a function generator with applied peak-to-peak voltage 1–20 Vpp over excitation frequencies 5–300 kHz. The device is mounted in a 3D-printed holder and positioned adjacent to an inverted microscope for observation.

Characterization setup: The capillary is immersed in liquid chambers seeded with 2.0 and 5.0 μm tracer particles. Streaming flows are imaged with light-sensitive and high-speed cameras. Particle image velocimetry (PIV) is performed with a custom MATLAB PIVlab script to quantify velocities and flow fields.

Streaming modes: Two steady acoustic microstreaming profiles are produced upon ultrasound activation: (i) helical/corkscrew-like streaming around the shaft, and (ii) frequency-specific 3D streaming at the tip. At 50 kHz and 20 Vpp, tracers exhibit out-of-plane circular motion around the capillary and spiral along the axis, with velocity increasing towards the narrow tip. The effective Reynolds number is ~0.07 (u ≈ 2.0 mm/s; particle diameter D ≈ 2.07 μm; water kinematic viscosity ν = 10−6 m²/s), indicating viscous-dominated flow; motion ceases immediately when the field is off. Streaming velocity scales approximately quadratically with applied voltage (u ∝ Vpp²) and decays with distance from the tip approximately as u ∝ 1/r². The circular flow direction is predominantly counter-clockwise, attributed to asymmetric bonding of the transducer.

Frequency-dependent tip vortices: By sweeping the driving frequency, distinct steady 3D vortex patterns appear at the tip, including butterfly-like four-vortex structures (e.g., 64.6 kHz, 140 kHz) and two-vortex patterns either symmetric about the tip (e.g., 69.5 kHz, 269 kHz) or off-centered by ~100 μm with rotated symmetry axes (e.g., 158 kHz, 239 kHz). Streaming strength (velocity and size) scales with acoustic power, while the pattern is set by frequency. Tip oscillation modes underlying these patterns are visualized in air at lower frequencies using high-speed imaging: elliptical motion at 6.8 kHz and translational at 7.8 kHz, supporting the hypothesis that frequency-dependent tip oscillations drive distinct streaming signatures.

Robotic control: The robotic arm is programmed via inverse kinematics and trajectory control to translate the tip vortex generator along arbitrary 2D/3D paths (e.g., rectangle, hourglass, letters “ETH”) at 1 mm/s with 1 s corner delays to ensure precision. The vortex position follows the capillary tip, enabling spatially programmable microvortex actuation.

Micropumping: The RAEE is immersed in a 3D-printed PDMS-based spiral channel. Pumping is achieved by placing the oscillating tip adjacent to one sidewall, breaking vortex symmetry, and generating net unidirectional flow. Operating at 134 kHz and 10 Vpp, mm/s-scale throughflow is induced, visualized by 6 μm tracers. Boundary-layer-driven acoustic streaming near the oscillating tip produces outer streaming flows that interact asymmetrically with channel walls to yield net transport. Flow rate is tunable via voltage (quadratic scaling) and stops immediately when acoustics are off.

Selective particle trapping (summary from results excerpts): Acoustic radiation forces generated by the vibrating capillary can trap larger microparticles; at higher voltages, 10 μm particles arrange into equidistant bands along the capillary corresponding to standing-wave pressure nodes, with measured spacing 270 ± 42 μm. Particles at nodes execute circular motion around the capillary circumference. Trapping limits depend on the balance of acoustic radiation and streaming forces; exact analytical force expressions for the vibrating capillary are not available.

Zebrafish embryo trapping: Anesthetized zebrafish embryos (120 hpf) are manipulated in liquid. The capillary is positioned within millimeters, then activated (80 kHz, 20 Vpp), pulling the embryo to the tip via dominant radiation forces (volume scaling). The swim bladder exhibits strong attraction. The embryo’s translational speed increases rapidly as it approaches the tip, scaling approximately with the fourth power of distance. Deactivating acoustics releases the embryo without visible harm. Embryos lacking swim bladders are also attracted.

Viscous droplet merging and mixing: Droplets of glycerol (black) and rhodamine solution (red) placed ~100 μm apart exhibit negligible passive diffusion. With the tip positioned on rhodamine and actuated at 44.9 kHz, 20 Vpp, the robotic arm translates across droplets; tip vortices capture and transfer rhodamine into glycerol, causing droplet merging and vigorous mixing, producing a sharp interface between mixed and unmixed regions. Uniform mixing is achieved within milliseconds locally.

Automated 96-well viscous mixing: A 96-well plate is filled with glycerol, then 20 μL rhodamine solution is added gently. After homing, the robot positions the tip 10 mm above the first well, lowers into the well, and executes a figure-8 path (each loop ~1 mm diameter) while actuated, stirring the viscous fluid. After 40 s, the tip is lifted to 10 mm above the well and moved to the next target well; the sequence repeats until completion, then returns to the initial position. Because vibration amplitude increases towards the tip, stirring is strongest near the bottom, avoiding solid-body rotation and enhancing mixing depth. The path and timing algorithm are pre-programmed for high-throughput operation.

• Integration: Demonstrated the first acoustofluidic end effector mounted on a robotic arm, providing a programmable interface between macro-robotics and microscale liquid/particle manipulation.
• Two streaming modes: (i) Helical/corkscrew streaming along the capillary shaft and (ii) frequency-specific 3D tip vortices (two- and four-vortex patterns), stable and repeatable for a given frequency.
• Scaling laws: Streaming velocity scales approximately as u ∝ Vpp² with applied voltage; velocity decays as u ∝ 1/r² with distance from the tip. The flow is viscous-dominated (Re ≈ 0.07). Predominantly CCW circular flow around the capillary due to asymmetric bonding.
• Micropumping: By placing the oscillating tip near a channel wall (e.g., spiral channel) at 134 kHz, 10 Vpp, mm/s-scale net flow is generated; flow rate is voltage-tunable and stops instantly when acoustics are off.
• Selective trapping: Larger microparticles can be trapped by radiation forces; at higher drive, 10 μm particles self-organize into axial bands at pressure nodes with spacing 270 ± 42 μm and circulate around the capillary at fixed stations, indicating a standing wavefield along the capillary.
• Zebrafish embryo handling: Anesthetized 120 hpf embryos are robustly trapped at the tip at 80 kHz, 20 Vpp; the swim bladder shows strong attraction. Embryo translational velocity increases with the fourth power of distance to the tip; embryos are released unharmed when acoustics are off. Embryos lacking swim bladders are also attracted.
• Viscous droplet operations: Tip vortices capture and transfer rhodamine into a glycerol droplet during robot-guided translation at 44.9 kHz, 20 Vpp, merging droplets and achieving rapid, localized uniform mixing in milliseconds with sharp mixed/unmixed interfaces.
• Automated high-throughput mixing: In a 96-well plate, figure-8 tip paths (1 mm loops) for ~40 s per well achieve effective mixing of viscous glycerol/rhodamine solutions. The approach ensures strong bottom stirring due to higher tip vibration amplitude near the tip.
• Cost and versatility: The end effector costs under $20 to fabricate and can be mounted on standard robotic arms, enabling accessible automation.

By coupling stable, frequency-programmable capillary oscillations with robotically controlled positioning, the RAEE addresses the long-standing gap between macro-robotic automation and microfluidic functionality. The system delivers multifunctional operations—pumping, selective trapping, droplet merging, and viscous mixing—within open and enclosed environments, reducing reliance on specialized channel geometries or enclosed devices. The observed scaling behaviors (u ∝ Vpp²; u ∝ 1/r²; viscous dominance) and frequency-selective tip vortex signatures provide predictable control knobs for tuning manipulation strength and spatial extent. Robotic programmability enables complex trajectories and high-throughput workflows (e.g., well-plate mixing), while instantaneous on/off control affords precise temporal modulation of flow and forces. These capabilities suggest broad relevance to sample preparation, bioassays, and microscale manufacturing, where automated, gentle, and reagent-efficient manipulation is valuable.

This work introduces a low-cost, capillary-based acoustofluidic end effector integrated with a robotic arm, enabling programmable and automated microscale liquid and particle handling. The device generates two robust streaming modalities—helical shaft streaming and frequency-dependent 3D tip vortices—to accomplish liquid pumping, selective particle and embryo trapping, droplet merging, and efficient mixing of viscous fluids, including automated well-plate workflows. The approach forms a practical interface between macro-robotics and microfluidics, with immediate utility in chemistry, biology, and life sciences. Future research will include detailed 3D characterization and modeling of tip-induced flows, advanced 3D microparticle manipulation and tissue-engineering patterning, acoustically induced poration/transfection, multi-capillary modular systems with programmable streaming profiles, and bi-directional pumping by frequency tuning and wall positioning. Integration with computer vision could enable automated, size-dependent embryo sorting and broader closed-loop manipulation.

• Pumping directionality: The present study demonstrates unidirectional pumping; achieving bi-directional flow will require control of vortex direction via frequency tuning or device-wall positioning.
• 3D manipulation: While dynamic, frequency-specific 3D tip flows are shown, comprehensive 3D microparticle manipulation remains to be developed.
• Modeling and measurement: Detailed numerical models of capillary oscillation modes and corresponding 3D streaming are not yet provided; direct high-frequency tip oscillation imaging is challenging. There is no analytical expression for radiation forces from a vibrating capillary. Velocity estimates near the tip have higher uncertainty due to limited fluorescent sensitivity of high-speed imaging.
• Generalizability and precision: Manipulation precision depends on the robotic arm’s accuracy and control; performance may vary with different robotic platforms.
• Selectivity boundaries: The minimum particle size for stable trapping is not analytically established; trapping depends on the balance of radiation and streaming forces and drive conditions.