Enhanced particle separation through ultrasonically-induced microbubble streaming for automated size-selective particle depletion

Abstract

The present work performed in the Institute for fluid mechanics and aerodynamics, at the Bundeswehr University Munich, describes how the researchers used our plasma treater in their microfluidic piezoelectric system to achieve automated size-selective particle depletion in continuous microchannel flow.

The research summary written by Dr Amirabas Bakhtiari is based on the peer-reviewed article “Enhanced particle separation through ultrasonically-induced microbubble streaming for automated size-selective particle depletion” by Amirabas Bakhtiari, and Christian J. Kähler. Their work was recently published in RSC Advances 2024, 14(4), 2226-2234.

In this research, we introduce an automated approach for achieving size-selective particle depletion within microchannels. Our method stands out for its label-free, sheath-free, and cost-effective characteristics. It integrates continuous Poiseuille flow with microbubble streaming, facilitating the manipulation of particles in an automatic or semi-automatic fashion at regular intervals.

Larger particles are retained near the microbubble, offering options for subsequent removal through a designated waste outlet or accumulation within a collection chamber for future analysis or manipulation. Unlike conventional techniques, our method confines the target particles in vortices adjacent to the microbubble while the primary fluid continues to flow through the microchannel. Consequently, these particles are expelled within a few milliseconds, conserving the primary fluid and substantially reducing fluid wastage.

Our analysis encompassed various critical aspects of the study. This included rigorous statistical examination, flow characterization using volumetric micro-Particle Tracking Velocimetry (PTV), and high-frequency micro PTV for observing flow field transitions. We also evaluated the system’s particle trapping capabilities across different sizes using a proprietary algorithm, and investigated the z-axis distribution of both incoming and escaped particles using volumetric micro PTV.

The invaluable insights derived from this analysis played a crucial role in refining the system and optimizing its operational parameters. This allowed us to achieve peak efficiency under diverse conditions, encompassing varying particle sizes, flow rates, and seeding densities.

Introduction | Microbubble streaming, revolutionizing particle separation in science and medicine

Particle separation techniques play a pivotal role across various domains including biology, medicine, and industry. In industrial settings, size-selective particle removal contributes to recycling efforts by extracting valuable metals from electronic waste, enhances mineral extraction in mining operations, and ensures water quality in wastewater treatment facilities.

However, the significance of particle separation extends far beyond industrial applications. In the biomedical field, it plays a crucial role in isolating rare cells such as circulating tumor cells (CTCs) from blood samples, thereby facilitating cancer research and early detection. Furthermore, size-selective particle removal enhances medical diagnostics by enabling rapid blood analysis through microfluidic devices, facilitating precise cell counting, and differentiation for point-of-care testing. In microbiology, the isolation of bacteria and microorganisms by size is essential for pathogen detection and environmental monitoring applications.

This research endeavors to propose the utilization of acoustically actuated microbubbles as a non-invasive and biocompatible method for capturing and separating microparticles in diverse scientific and medical contexts. By leveraging acoustic excitation, these microbubbles induce oscillatory and secondary flows within microchannels, enabling precise manipulation of particle or biological entities. Our proposed technique holds immense potential for advancing non-invasive cellular separation methods, thus offering significant implications for a wide array of scientific and medical applications.

Aims

Enhance particle separation efficiency using microbubble streaming through the development of a novel design.
Develop a technique that eliminates the requirement for buffer flow in the particle separation process.
Explore the fundamentals of microbubble streaming through statistical examination and flow characterization using volumetric micro PTV, and high-frequency micro PTV.
Investigate the z-axis distribution of particles using volumetric micro PTV to gain insights into high-concentration particle migration within microbubble streaming.

Experiment Setup

The microfluidic system depicted in the illustration (Fig. 1) encompasses a sophisticated experimental setup featuring microchannels and precise flow control mechanisms. Within this setup, the focal point is a polydimethylsiloxane (PDMS) microchannel, crafted using the conventional soft lithography technique. To ensure optimal performance, the microchannel is meticulously bonded to a glass slide previously coated with a 1-mm-thick PDMS film.

The enhancement comes through the application of surface functionalization, a process achieved using a PDMS plasma bonder sourced from Elveflow, headquartered in France. This advanced equipment guarantees that all channel walls are uniformly composed of PDMS, a crucial step in achieving seamless integration and robust performance within the microfluidic device. The utilization of Elveflow’s PDMS plasma bonder not only bolsters the adhesion between the PDMS film and the glass substrate but also fortifies the structural integrity of the microfluidic system. Moreover, this treatment optimizes the wettability properties of the surfaces, ensuring unimpeded fluid flow while minimizing the risk of leakage or detachment during operational cycles.

Later, the flow of the aqueous sample solution into the channel is meticulously managed using a syringe pump in tandem with a pressure regulator, ensuring the fidelity of experimental conditions across iterations.

In the presence of microstreaming, smaller particles act as passive tracers, while larger particles are entrapped near microbubble surfaces (Fig. 2). Our study focuses on extracting these larger particles from a two-phase flow, where microstreaming confines them close to the microbubble while smaller particles escape. After deactivating the piezoelectric element, larger particles are propelled downstream by the main flow. An automated side channel then briefly opens, discharging the larger particles into a waste or collection chamber for further analysis.

Key Findings | Optimizing operational parameters for peak efficiency

Flow characterization

Initially, we performed micro-PTV analysis on 15-micrometer particles within a flow combining Poiseuille flow (left to right) and microbubble streaming generated by a piezoelectric transducer. The transducer operated at varied voltage amplitudes of 70, 35, and 20 vpp, all at a frequency of 33 kHz. The flow field, as depicted in the figure, is divided into distinct topologies by the combination of microbubble streaming and Poiseuille flow, resulting in two primary regions separated by a dividing line or separatrix.

Particles located above the separatrix line bypass counter-rotating vortices and exit downstream of the bubble. Conversely, particles beneath the separatrix approach the bubble surface. Increasing the amplitude of bubble excitation enlarges vortex sizes, leveling the separatrix towards the opposite bubble wall.

These experimental results vividly demonstrate that microstreaming induced by the excited piezo transducer (Fig. 3A, with an amplitude of 66 vpp) effectively aggregates particles of all sizes towards the bubble.

Figure 3: The tracking of 15-micrometer particles within a Poiseuille flow (left to right) is accompanied by microbubble streaming induced by continuous excitation of the piezoelectric transducer at frequencies of 33 kHz. The three cases, (a), (b), and (c), correspond to excitation amplitudes of 70, 35, and 20 vpp respectively. In case (a), particles are attracted and concentrated near the microbubble. Conversely, cases (b) and (c) exhibit decreased amplitudes, creating a bypass for particles positioned above the red separatrix line.

High-frequency micro-PTV

Understanding the response time between activating the piezoelectric element and redirecting the particles is crucial for developing the control system. In this investigation, we use high-frequency microparticle tracking velocimetry (24,000 frames per second) to analyze the dynamic shift in the flow field (Fig. 4). This transition occurs from pure Poiseuille flow to a combination of Poiseuille flow (left to right) with the introduction of microbubble streaming upon activation and deactivation of the piezoelectric element.

Our high-frequency micro-PTV analysis, lasting 125 ms before (depicted in black) and 416 ms after (depicted in color) initiating bubble excitation via the piezoelectric element, is illustrated in Fig. 4. The immediate alteration in particle displacements is evident after the initial cycle of bubble activation. Closer inspection reveals that the newly adopted trajectories exhibit perfect synchronization, without any temporal delay. Similarly, when transitioning from the superposition flow to pure Poiseuille flow, we observe an immediate cessation of lateral particle movement without delay. The particles maintain their respective lateral positions until reaching the end of the microchannel.

Particle trapping capacity

Figure 5 illustrates particle trapping efficiency across different sizes (5, 10, and 15 micrometers). The best performance was seen with 10-micrometer particles, showing a 100% success rate with no leakage until the critical limit. However, as particle input increased, leakage rose until reaching an equilibrium between input and output particles. The clean trapping capacity decreased to 70 particles for 5-micrometer and 60 particles for 15-micrometer sizes. Larger particles filled the torus faster, leading to quicker interactions, while 5-micrometer particles were more prone to leakage due to their proximity to critical perigee size as trapped concentrations intensified within vortices.

Figure 5: The visual representation demonstrates particle trapping efficiency based on size (5, 10, and 15 micrometers) under uniform microbubble excitation at 33 kHz frequency and 66 Vpp amplitude. The main flow velocity remains constant at 200 micrometer/s. Each line in the graph represents a single experimental run. Notably, trapping performance is notably better for 10-micrometer particles (in red) compared to both 5-micrometer particles (in green) and 15-micrometer particles (in blue).

Leaked particles Z-axis distribution analysis

The volumetric micro-PTV results illustrate the spatial distribution of particles with diameters of 5, 10, and 15 micrometers along the z-axis, aiming to understand particle leakage (Fig. 6). The 10 and 15-micrometer particles cluster near the channel wall with a standard deviation of approximately 10-20 micrometers, indicating significant shear-induced lift force. In contrast, 5-micrometer particles disperse throughout the channel depth.

This suggests that during particle trapping, larger diameter particles (15 and 10 micrometers) experience stronger shear-induced lift forces, leading them closer to the channel wall for a common exit point along the z-axis. The exit process is faster for 15-micrometer particles due to the quicker filling of space, facilitating faster particle-particle interactions.

Figure 6: Mean and standard deviation of the z-axis distribution for particles with diameters of 5, 10, and 15 micrometers. A dashed black line from the microchannel's bottom wall marks the area inaccessible to particles due to their finite size. Incoming particles (blue) show a uniform distribution along the z-axis. In contrast, leaked particles of 10 and 15 micrometers (red) cluster near the channel wall, while 5-micrometer particles disperse evenly across the channel depth, lacking the wall-focused behavior seen in larger particles.

Conclusion | An optimized size-selective particle depletion in continuous microchannel flow

In summary, this study introduces and validates an automated method for achieving size-selective particle depletion in continuous microchannel flow. Particle separation and removal are facilitated using a PDMS microchannel and piezoelectric technology, providing a simple yet effective alternative to expensive devices like flow cytometers.

The method effectively retains larger particles near microbubbles without requiring buffer flow, allowing for subsequent removal or collection. Unlike conventional techniques, it keeps target particles within vortices near microbubbles while the primary fluid continues to flow through the microchannel, enabling rapid particle ejection, minimizing fluid wastage, and preserving the primary fluid.

Our analysis covers various critical aspects, including statistical examination, micro PTV flow characterization, and high-frequency micro PTV for flow field observations. We evaluated particle trapping capabilities across different sizes using a proprietary algorithm and explored z-axis particle distribution using volumetric micro PTV. Insights from this data refined the system and optimized operational parameters for peak efficiency under diverse conditions, accommodating varying particle sizes, flow rates, and seeding densities.

The system’s flexibility allows for customization to meet practical needs, accommodating different flow conditions, particle sizes, and distribution rates. This adaptability ensures its applicability in various scenarios, highlighting its potential for widespread utility and innovation in microfluidics and particle manipulation.