Session R30: Micro/Nano scale Flows: Bubbles and Particle Applications

Oscillatory electro-inertial microfluidics for tunable particle manipulation

Conventional microfluidic techniques that make use of only one force (i.e., passive or active) for particle manipulation are limited in their abilities by their low throughput or lack of tunability when it comes to complex heterogeneous samples. To overcome these limitations, the emerging field of multiphysics microfluidics integrates two or more techniques on a single device either as cascaded connections or by physical coupling. However, the richness and complexity of multiphysics microfluidic phenomena have not been thoroughly investigated or understood. Here, we present results on the electro-inertial migration of particles in coupled synchronous oscillatory pressure-driven and electrokinetic flows. Using our apparatus, oscillatory flows create an "infinite" channel length enabling the investigation of electro-inertial migration velocity and focusing positions of a range of synthetic and biological particles. We quantify the migration velocity based on relevant system parameters such as particle size, oscillatory flow frequency and amplitude, and electrokinetic mobility. By doing so, we contribute to the comprehension of fundamental mechanisms underlying electro-inertial lift and establish a versatile platform for complex particle manipulation.   

Active Heat Transfer Fluids: Enhancement of Convective Heat Transfer by Self-Propelled Particles

Liquid coolants are critical components of many modern technologies, including hybrid electric vehicle batteries, solar receivers, and cooling systems for electronics. Sustaining the growth of these technologies while minimizing their carbon footprint requires coolants that transfer heat efficiently. Since the 1990s, there has been significant research into the use of suspended nanoparticles to improve the heat transfer performance of liquids. The resulting suspensions, known as nanofluids, generally have higher thermal conductivities than the liquids alone, but the improvements provided by nanoparticles are limited. In this work, we investigate the use of self-propelled particles (SPPs), which move autonomously in liquids, and especially the "micro-stirring" they cause in the surrounding fluid, to enhance the transport of heat through liquids. We hypothesize that the enhancement depends primarily on the SPPs' size, speed, material, and volume fraction. We conducted simulations to demonstrate the efficacy of this approach. We also demonstrate this phenomenon experimentally by placing an SPP suspension in contact with a heated surface; for a constant heat flux, a more effective coolant will result in a lower surface temperature. Thus, the heater surface temperature is a figure of merit for cooling performance. In this study, we investigate the heat transfer enhancement performance of several different SPP designs in various geometries. The results will inform the prospective use of coolants containing SPPs, which we term "active heat transfer fluids" (AHTF), in energy and environmental applications.   

Liquid Film Flow Characterization in Microfluidic Oscillating Heat Pipes Using Particle Image Velocimetry

Due to continuous reduction of the size yet increased performance of electronic systems, increased heat generation has been a critical issue. To solve this issue, oscillating heat pipes (OHP) have emerged as an effective cooling solution. OHPs are preferred in some scenarios over conventional heat pipes as the latter lose their efficiency when the device thickness decreases below 2-3 mm. OHPs operate on the basis of a two-phase system of vapor and liquid. The interaction between the liquid and vapor creates a thin film region between the vapor bubble and the walls of the oscillating heat pipe, whose role is currently not well understood. Therefore, this study focuses on the investigation of the thin film region. Thermocouples and particle image velocimetry are used for analyzing the temperature profile and fluid flow patterns across the thin film region. The OHP is fabricated using standard photolithography and deep etching, where microchannels are etched in a silicon wafer with a glass wafer bonded on top. Ethanol is used as a working fluid. The study provides valuable insights into the role of thin films in heat transport processes, which will be significant to better understand the fundamental mechanisms and improve the effectiveness of OHPs.   

Measurement of particle behavior in micro/nano channels using defocusing nano-PIV

With the recent progress of micro/nanotechnology, understanding particle transport in small spaces with dominant surface effects becomes important. For this issue, micro-particle image velocimetry (micro-PIV) has been developed, but the spatial resolution is insufficient for study on spaces smaller than micrometers. We have developed defocusing nano-PIV that determines particle positions with spatial resolution overcoming the optical diffraction limit using defocused particle images with diffraction rings due to spherical aberration. However, the measurement accuracy is still insufficient to obtain results that reflect actual phenomena.

The present study improved the spatial resolution of defocusing nano-PIV by determination of the positions of center peak and diffraction ring of the particle image. In our previous work, the spatial resolution in streamwise and depthwise directions were respectively 40.0 nm and 42.6 nm, but in this study, they were 7.4 nm and 9.9 nm. We archived the spatial resolution similar to the dispersion of particle diameter. Based on these improvements, we measured the flow velocity profile in a channel with a depth of 2500 nm for particles of 200 nm diameter, and the result was close to the theoretical flow velocity distribution. Thus, we succeeded in measurement of the particle behavior in micro channels that reflects actual phenomena. In the future, we will measure the particle behavior in nanochannels.   

Nanoparticle Focusing via Acoustofluidics for Characterizing Low-Concentration Viral Particles.

This presentation introduces a combined method of nanoparticle manipulation and characterization using acoustofluidics and Raman spectroscopy. Due to its non-intrusive nature and absence of labeling requirements, Raman spectroscopy is extensively used to analyze biological entities such as cells and bacteria. However, when dealing with viral particles within the diameter range of 50 nm to 400 nm, Raman spectroscopy requires higher concentrations due to the small volume of these particles. We employed a 2D acoustofluidic device to overcome this limitation to locally enhance the concentrations of nanoparticles. We successfully improved the detection limit for low concentrations by three orders of magnitude by integrating Raman spectroscopy with an acoustic microfluidic device. In this presentation, we will also demonstrate the practical application of this system by successfully measuring Raman spectroscopy from low-concentration Ebola viral particles. Our work presents a real time monitoring approach for characterizing nano- and bio-particles, shedding light on microfluidics for drug transportation, biosensing, pharmaceutical production, environmental surveillance, and Process Analytical Technology (PAT).   

Mitigation of clogging in a microfluidic array via pulsatile flows

Clogging is a common obstacle encountered during the transport of suspensions and represents a significant energy and material cost across applications, including water purification, irrigation, biopharmaceutical processing, and aquifer recharge. Pulsatile pressure-driven flows can help mitigate clogging when compared to steady flows. Here, we study experimentally the influence of the amplitude and the frequency of pulsation on clog mitigation in a microfluidic array of parallel channels using a dilute suspension of colloidal particles. We combine flow rate measurements with direct visualizations at the pore scale to correlate the observed clogging dynamics with the changes in flow rate. We observe that the rearrangement of particles when subject to a dynamic shear environment can delay the clogging of a pore or even remove an existing clog. However, this benefit is drastically reduced at too low frequencies as the pulsatile timescale becomes too large compared to the timescale associated with the clogging. Our experiments also reveal that large amplitude pulsations, which result in periodic flow reversal, can accelerate the clogging of the system through the interaction of adjacent pores. The present study demonstrates that pulsatile flows are a promising method to delay clogging at both the pore and system scale.   

Bubble racing in a Hele-shaw cell

We study theoretically and experimentally the propagation of inviscid bubbles in a Hele-Shaw cell under a uniform background flow, in the regime where each bubble remains approximately circular. New experimental results for the velocity of an isolated bubble are found to agree well with theoretical predictions. For a train of three collinear bubbles, we observe that the middle bubble closes in on the leader, again in agreement with theory. Finally for a system of two non-identical bubbles, we find that the larger bubble overtakes the smaller one, and they avoid contact by rotating around each other while passing.