Micro Particle Image Velocimetry Research Articles

Facile compliance-based pump for blood physiometer

The biomechanical properties of blood are considered promising label-free biomarkers for early disease detection. Disposable pumps have been suggested as replacements for bulky and expensive syringe pumps. However, they have limitations, including initial air bubble removal, simple stop-and-run flow control, and quantification of many rheological properties. In this study, a compliance-based pump (CP) is developed by fitting a blood-loaded syringe, an air-compliance unit, and a needle into each port of a three-way valve. When blood is loaded into the microfluidic channel from the CP, the initial air bubbles are removed from the channels. By manipulating the three-way valve, blood flow is stopped immediately. Red blood cell (RBC) aggregation index (AI) is obtained by analyzing microscopic blood images. The air-compliance unit induces a transient blood flow. The time-resolved micro-particle image velocimetry technique is employed to obtain the blood velocity. The flow rate and time constant are obtained by assuming the flow rate as Q (t) = Q1 exp(−t/λ1) + Q2 exp(t/λ2) and conducting nonlinear regression analysis. The proposed method is employed to obtain the properties (AI, Q1, Q2, λ1, and λ2) of suspended bloods (hematocrit = 30%–70%, 5–30 mg/ml dextran solution, and heat-shocked RBCs). It is also used to detect four different types of suspended blood prepared by adding two types of RBCs (normal or hardened RBCs) to two types of diluents (1 × phosphate-buffered saline or dextran). In conclusion, the proposed method can be used to detect differences in suspended blood by manipulating the CP and consistently analyzing microscopic blood images.

Relating ciliary propulsion morphology and flow to particle acquisition in marine planktonic ciliates I: the tintinnid ciliate Amphorides quadrilineata

Abstract The marine tintinnid ciliate Amphorides quadrilineata is a feeding-current feeder, creating flows for particle encounter, capture and rejection. Individual-level behaviors were observed using high-speed, high-magnification digital imaging. Cells beat their cilia backward to swim forward, simultaneously generating a feeding current that brings in particles. These particles are then individually captured through localized ciliary reversals. When swimming backward, cells beat their cilia forward (=ciliary reversals involving the entire ring of cilia), actively rejecting unwanted particles. Cells achieve path-averaged speeds averaging 3–4 total lengths per second. Both micro-particle image velocimetry and computational fluid dynamics were employed to characterize the cell-scale flows. Forward swimming generates a feeding current, a saddle flow vector field in front of the cell, whereas backward swimming creates an inverse saddle flow vector field behind the cell; these ciliary flows facilitate particle encounter, capture and rejection. The model-tintinnid with a full-length lorica achieves an encounter rate Q ~29% higher than that without a lorica, albeit at a ~142% increase in mechanical power and a decrease in quasi-propulsive efficiency (~0.24 vs. ~ 0.38). It is also suggested that Q can be approximated by π(W/2 + l)2U, where W, l and U represent the lorica oral diameter, ciliary length and swimming speed, respectively.

Initial contact and spreading of a non-Newtonian drop on a solid surface

In this work, the very initial contact and spreading of drops of two non-Newtonian fluids on a hydrophilic stainless-steel surface were comparatively investigated. The fluids employed were respectively a polymer solution of polyacrylamide (PAAm) and an aqua dispersion of fine graphene oxide (GO) particles. Experimental studies involved visualizing and characterizing the spreading width over time on the substrate surface. A more precise investigation into the initial contact and spreading of non-Newtonian pendant drops on a solid surface was performed through a home-made ultra-high-speed direct current (DC) electrical device with an acquisition frequency up to 1.25 MHz (time interval of 0.8 μs), a high-speed camera and a home-developed high-speed micro-particle image velocimetry (micro-PIV). The competing mechanisms of related forces: inertial, viscous, and capillary forces were probed to lead to different scaling laws.

Conjugate heat transfer analysis of sidewall-heated rectangular microchannels with different bottom wall materials

In this study, the conjugate heat transfer in a microchannel under four sidewalls (left, right, top, and bottom wall facing upstream) heating conditions is investigated experimentally and numerically. The microchannel has a width of 500 μm and a depth of 100 μm. Micro-Particle Image Velocimetry (Micro-PIV) and Temperature Sensitive Paint (TSP) are respectively applied to measure the velocity and temperature fields at a Reynolds number (Re) of 20. The experimental results demonstrate that the temperature gradient exists in both the left, right, and bottom wall, indicating the existence of conjugate heat transfer. Thus, 3D conjugate heat transfer simulations are subsequently conducted. Good agreements are attained between numerical and experimental data only with the non-ideal boundary condition. Otherwise, there is a significant discrepancy between them. Further parametric studies on bottom wall to fluid thermal conductivity ratio (kb/kf) and Re show that the fully developed overall Nusselt number (Nu‾) in the conjugate heat transfer case is approximately 4.55, much higher than the analytical solution 0.87 without accounting for conjugate heat transfer. As kb/kf is varied from 0.21 to 6.48, there exists a critical kb/kf=1.79, before and beyond which the total heat flux respectively increases and decreases with kb/kf. However, Nu‾ is a monotonically increasing function of kb/kf, with a peak value of 5.75 occurring at kb/kf = 6.48.

On flow disturbances caused by pressure taps in highly elastic flows around a microfluidic cylinder

The objective of this work is to characterize the onset of laterally asymmetric flow of viscoelastic solutions around a confined microfluidic cylinder, which was encountered in a recent study [Rodrigues et al., J. Non-Newtonian Fluid Mech. 289, 104406 (2020)]. To this end, two non-Newtonian fluids were employed in the same micro-geometry. Two microchannels were studied, both with a cylinder of diameter 75 μm, aspect ratio (channel height over width) of 0.37, and blockage ratio (cylinder diameter over channel width) of 0.28, differing only on the width of the pressure taps, located 500 μm up- and downstream from the respective cylinder face, on opposing walls. The working fluids consist of two poly(ethylene oxide) solutions: an elastic weakly shear-thinning fluid and an elastic shear-thinning fluid. Micro-Particle Image Velocimetry and streak imaging techniques were used to evaluate the flow over a Weissenberg number range 100≤Wi≤500, while maintaining a low Reynolds number, Re < 1. The elastic shear-thinning solution showed laterally asymmetric flow past the cylinder with both pressure tap designs, while with the weakly shear-thinning solution asymmetric flow was only observed with the wider pressure tap intake. In both cases, the fluids preferentially chose the cylinder/wall gap opposing the upstream pressure tap, which was found to influence the flow greatly, seemingly associated with time-dependent flow and possibly the lateral flow asymmetry itself. This work brings to light the necessary compromise between optimal pressure tap design for quality pressure measurements and minimal flow interference, due to the increased susceptibility of elastic microfluidic flows to flow perturbations.

Co-effect of hydrophobicity and cavities on flow characteristics at microscale

Flowing characteristics of drag reduction are experimentally and numerically investigated with a combination of hydrophobic coating and various cavities in polydimethylsiloxane microchannels. Two typical types of cavities with different geometrical parameters, rectangular and triangular, are studied with a wide range of Reynolds numbers from 0 to 300. A promising hydrophobic processing method is proposed based on a comprehensive performance on surface morphology, water repellency, and structural distortion. In this method, hydrofluoric acid (40 wt. %) is adopted and the corrosion time is set to 5 min. The velocity field and streamlines are acquired by the micro-particle image velocimetry system and numerical models to explain the flow patterns in particular. The slip length is measured as 13.38 μm in the hydrophobic straight channel. For rectangular cavities, the drag reduction rate reaches nearly 14.1% under no-slip condition and 33.2% under slip condition. A critical turning point of the co-effect is found by numerical results when the slip length is about 15 μm, which is also determined by the cavities. The convergent and divergent angles of triangular cavities play a critical role in the pressure drop due to the competition of the vortex and flow impingement. A nonlinear model is developed based on the numerical results to predict Poiseuille number with the relevant important variables for a two-dimensional microchannel. Our results reveal the fundamental physics of flowing characteristics with the co-design of hydrophobicity and microstructures, predicting a composite design method for widespread applications in microfluids.

High-resolution microscale velocity field measurement using light field particle image-tracking velocimetry

Light field microparticle image velocimetry (LF-μPIV) can realize the three-dimensional (3D) microscale velocity field measurement, but the spatial resolution of the velocity field is low. Therefore, this study proposes a high-resolution LF particle image-tracking velocimetry (PIV–PTV) in combination with a cross-validation matching (CVM) algorithm. The proposed method performs motion compensation for the distribution of particle center position based on the low-resolution velocity field achieved by PIV and then conducts the CVM on tracer particles with the nearest neighbor method. The motion compensation reduces the particle displacement during the matching, while the CVM reduces the impact of missing particles on the matching accuracy. Thus, the proposed method enables precise tracking of individual particles at higher particle concentrations and improves the spatial resolution of the velocity field. Numerical simulations were conducted on the 3D displacement field reconstruction. The influence of interrogation window size, particle diameter, and concentration was analyzed. Experiments were conducted on the microscale 3D velocity field within the microchannel with right-angle bends. Results indicate that the proposed method provides the high-resolution measurement of the microscale 3D velocity field and improves the precision of the velocity field compared to the PTV at higher particle concentrations. It demonstrates that the proposed method outperforms PIV by 26% in resolution and PTV by 76% in precision at a higher particle concentration of 1.5 particles per microlens.

Experimental analysis of pulsatile flow effects on flow structure in transitional-type cavity

Studies combining flow separation and pulsatile flow phenomena are inherent in the field of biomedicine and have many engineering and biomedical applications. In this study, the pulsatile flow structure is investigated experimentally based on a transitional-type cavity with a length-to-depth ratio of 8 using a microparticle image velocimetry system. A pulsatile flow is generated by pulsating the inlet pressure in sinusoidal pulses using a pressure control unit. Stationary flow and four sets of pulsatile parameters are investigated: two pulsation amplitudes (A = 0.15 and 0.60) and two pulsation frequencies (f = 0.5 and 1 Hz) in the Reynolds range of 50–2000. The recirculation flow dynamics in a cavity are analyzed by investigating the effect of pulsations on the flow structure and statistical flow parameters such as vorticity, shear rate, and turbulence intensity. The analysis shows that the effect of the pulsation amplitude on the recirculation zone dynamics is more prominent than that of the pulsation frequency. The magnitude of the recirculation zone reduction achieved by the pulsatile flow is inversely proportional to the pulsation amplitude. A reduction in the recirculation zone length decreases the shear rate distribution along the cavity. Additionally, an analysis into the turbulence intensity shows that effect of pulsations is negligible when the flow approaches the turbulent flow regime.

The Path from Nasal Tissue to Nasal Mucosa on Chip: Part 2-Advanced Microfluidic Nasal In Vitro Model for Drug Absorption Testing.

The nasal mucosa, being accessible and highly vascularized, opens up new opportunities for the systemic administration of drugs. However, there are several protective functions like the mucociliary clearance, a physiological barrier which represents is a difficult obstacle for drug candidates to overcome. For this reason, effective testing procedures are required in the preclinical phase of pharmaceutical development. Based on a recently reported immortalized porcine nasal epithelial cell line, we developed a test platform based on a tissue-compatible microfluidic chip. In this study, a biomimetic glass chip, which was equipped with a controlled bidirectional airflow to induce a physiologically relevant wall shear stress on the epithelial cell layer, was microfabricated. By developing a membrane transfer technique, the epithelial cell layer could be pre-cultivated in a static holder prior to cultivation in a microfluidic environment. The dynamic cultivation within the chip showed a homogenous distribution of the mucus film on top of the cell layer and a significant increase in cilia formation compared to the static cultivation condition. In addition, the recording of the ciliary transport mechanism by microparticle image velocimetry was successful. Using FITC-dextran 4000 as an example, it was shown that this nasal mucosa on a chip is suitable for permeation studies. The obtained permeation coefficient was in the range of values determined by means of other established in vitro and in vivo models. This novel nasal mucosa on chip could, in future, be automated and used as a substitute for animal testing.

Dynamics of rigid achiral magnetic microswimmers in shear-thinning fluids

Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water.

Experimental analysis of flow topology and particle behavior in microcavities

Efficient hydrodynamic particle trapping requires the precise control of flow structures that serve as hydrodynamic traps. Microfluidic structures, such as microcavities of various shapes and sizes, can be applied as hydrodynamic traps, and the exerting force can be controlled by changing the geometrical parameters and flow regimes of the cavities. In this study, the capability of trapping microparticles in rectangular, semicircular, and triangular microcavities is experimentally investigated. The flow structure in microcavities with a length-to-depth ratio L/h of 1–4 for rectangular cavities and L/h = 1–2 for semicircular and triangular cavities at a Reynolds number ranging between 1 and 1000 is visualized using a microparticle image velocimetry system. Additionally, patches of 20 µm particles are visualized using a high-speed camera. Different flow phases, namely attached, transitional, and separated, are observed and correlated with the cavity parameters and flow regime. Additionally, qualitative flow parameters determining particle behavior, such as the vortex center location and temporal vortical stability, are analyzed. Finally, the particle behavior is correlated with the flow structure in the cavities, and guidelines for the development of the optimal design of micro- and nanofluidic devices are discussed.

The Fate of Chemical Species from a Sample Introduced into a Redox-Magnetohydrodynamics (R-MHD) Microfluidics Chamber: Influence of Diffusion within Flow Fields Near Pumping Electrodes and Walls and Under Different Experimental Conditions

Redox-magnetohydrodynamics (MHD) offers unique capabilities for pumping liquids on a small scale where other microfluidic pumps are ineffective or unsuitable. It can vary speed, change direction, stir and pump in a loop by controlling the magnitude and direction of ionic current, j, between two or more electrodes having desired geometries in the presence of a perpendicular magnetic field, B. This interaction generates a force, FB , orthogonal to j and B, following the right hand rule. The localized volume of liquid then moves in the same direction as FB due to momentum transfer. MHD is compatible with a variety of solvents and solution compositions. In principle, MHD does not require valves or channel sidewalls to move fluid in a path. For applications involving analysis or imaging of single entities, like nanoparticles, microbeads, and biological cells, the focus is on inspection of the entities in a specific region of moving fluid. However, in applications where there is a collection of species, such as a concentration of an analyte in a sample introduced into the fluid having a different composition, it is important to understand how the analyte disperses from both the fluid dynamics and diffusion. Numerical approximations were performed using COMSOL Multiphysics® to analyze the 3D velocity profiles generated experimentally by redox magnetohydrodynamics (R-MHD) microfluidics and to predict the fate of analyte molecules in a sample that is introduced into that fluid.Simulations were first compared to experimental data from parallel band, pumping electrodes, where uniform horizontal flow profiles are possible when sidewalls are far away. Experimentally obtained velocities involved an R-MHD chamber with a rectangular geometry of 30 mm x 17 mm formed from a cutout in a poly(dimethyl siloxane) (PDMS) gasket placed onto a microfabricated chip containing individually-addressable electrodes, filled with an electrolyte solution, and capped with a glass coverslip. The gasket thickness defined the chamber height. To generate an ionic current, an electronic current was applied between two poly(3,4-ethylenedioxythiophene) (PEDOT) modified, parallel band electrodes with dimensions of ~890 µm width and 15 mm length (along the x-direction), separated by a 2.76 mm gap. A magnetic flux density of 0.37 T was produced by a permanent magnet placed beneath the chamber. Astigmatism particle tracking velocimetry (APTV) was used to interrogate the fluid velocity between the electrodes where only a single camera was needed to capture all three velocity components in a 3D volume. This technique measures the astigmatic aberration to encode the out-of-plane position of a tracer particles in the measurement volume. In some experiments, microparticle image velocimetry (μPIV) was used to evaluate particle velocity by measuring displacement of tracer particles along horizontal planes. Velocity profiles with high spatial resolution from the APTV and µPIV measurements provided a detailed look at the effects on fluid motion in three dimensions as a function of applied current (from ± 50 µA to ± 1000 µA), electrolyte concentration (from 0.1 NaCl + 10% glycerol to keep the density constant, to 1.3 M NaCl + 0% glycerol), and chamber height (from 429 µm to 1690 µm). Simulated velocity profiles between electrodes in the x-direction are within 95.8 of those of APTV experiments involving variation in currents for a fixed chamber height and different chamber heights for a single applied current. In studies involving µPIV and variation of electrolyte concentrations, the magnitude of maximum fluid velocity is higher than for simulations. (We will present studies where this discrepancy is addressed.)Then, using simulations we “introduced” a sample, 20-µm across centered between pumping electrodes, with initial concentration of 0.1 M and diffusion coefficient of 8.75 x 10-6 cm2/s. Time-dependent evolution of molecule transport was superimposed on the steady-state R-MHD. The spread and direction of molecular species outside of the pumping path greatly depends on location of chamber sidewalls from the outer edges and ends of band electrodes.Diffusional and fluid dynamic spread of sample after introduction between pumping electrodes having concentric circular geometries was then studied by simulations. Here, fluid pumps in a loop. Wall and sample location and electronic current magnitude were taken into consideration. Knowledge from these configurations are of interest for chip-based, circular chemical separations, with an “infinite” number of plates and goal of higher separation efficiencies. Horizontal flow profiles for R-MHD at concentric ring electrodes are not uniform, because of unequal ionic current densities across the curved flow path. Results from these numerical approximations and implications for designing lab on a chip applications for pumping sample plugs by R-MHD microfluidics with and without side walls for chemical applications will be discussed.

Extensional rheometry of mobile fluids. Part II: Comparison between the uniaxial, planar, and biaxial extensional rheology of dilute polymer solutions using numerically optimized stagnation point microfluidic devices

Part I of this paper [Haward et al., J. Rheol. 67, 995–1009 (2023)] presents a three-dimensional microfluidic device (the optimized uniaxial and biaxial extensional rheometer, OUBER) for generating near-homogeneous uniaxial and biaxial elongational flows. Here, in Part II, the OUBER device is employed to examine the uniaxial and biaxial extensional rheology of model dilute polymer solutions, compared with measurements made under planar extension in the optimized-shape cross-slot extensional rheometer [OSCER, Haward et al. Phys. Rev. Lett. 109, 128301 (2012)]. In each case, micro-particle image velocimetry is used to measure the extension rate as a function of the imposed flow conditions, and excess pressure drop measurements enable estimation of the tensile stress difference generated in the fluid via a new analysis based on the macroscopic power balance for flow through each device. Based on this analysis, for the most dilute polymer sample tested, which is “ultradilute”, the extensional viscosity is well described by Peterlin’s finitely extensible nonlinear elastic dumbbell model. In this limit, the biaxial extensional viscosity at high Weissenberg numbers (Wi) is half that of the uniaxial and planar extensional viscosities. At higher polymer concentrations, although the fluids remain dilute, the experimental measurements deviate from the model predictions, which is attributed to the onset of intermolecular interactions as the polymer chains unravel in the extensional flows. Of practical significance (and fundamental interest), elastic instability occurs at a significantly lower Wi in uniaxial extensional flow than in either biaxial or planar extensional flow, thereby limiting the utility of this flow type for extensional viscosity measurement.

Experimental and numerical investigation of a density-driven instability during a horizontal miscible displacement

We performed an experimental and numerical investigation of a convective buoyancy-driven instability that arises during the injection of a denser miscible fluid into a less dense one in a rectilinear geometry. We visualized the instability using a shadowgraph technique, and we obtained quantitative information using micro-Particle Image Velocimetry. Numerical simulations provided further insights into the three-dimensional (3D) velocity field. We have shown that the instability only occurs above a certain Péclet number, Pe, depending on the Rayleigh, Ra, and Schmidt, Sc, numbers. We suggest scalings of the critical time, TC, and dimensionless wavelength, λ/h, of the instability, both of which increase with increasing Pe and Ra. Finally, we investigated the interactions of the instability vortices with each other and the geometry boundaries.

Flow Characteristics and Vortex Structure of Flow Across Micro Cylinders by Micro-PIV Measurement and CFD Simulation

The micro tube bank model is a commonly used structure in microelectro mechanical cooling systems. Flow across a micro tube bank is a basic benchmark for analyzing flow resistance and heat transfer capability. In this paper, the flow around a micro tube bank model was studied both experimentally and numerically. Micro-Particle Image Velocimetry technology was used to measure the flow field at different inlet flow rates. The laminar flow model, S-A model and standard k-ε model were used to calculate the flow field inside the micro tube bank model. By comparing the results from the three numerical models with the experimental results, there is a certain gap between them can be noted. However, it was found that the standard k-ε model is better than laminar model and S-A model in the comparison between numerical results and experimental results. In conclusion, the standard k-ε model is more suitable for the numerical calculation of the flow field around a micro scale tube bank model.

Experimental assessment of cardiovascular physiology in the chick embryo.

High resolution assessment of cardiac functional parameters is crucial in translational animal research. The chick embryo is a historically well-used in vivo model for cardiovascular research due to its many practical advantages, and the conserved form and function of the chick and human cardiogenesis programs. This review aims to provide an overview of several different technical approaches for chick embryo cardiac assessment. Doppler echocardiography, optical coherence tomography, micromagnetic resonance imaging, microparticle image velocimetry, real-time pressure monitoring, and associated issues with the techniques will be discussed. Alongside this discussion, we also highlight recent advances in cardiac function measurements in chick embryos.

Characterizing Internal Flow Field in Binary Solution Droplet Combustion with Micro-Particle Image Velocimetry

Droplet internal flow participates in liquid-phase mass transfer during multicomponent solution droplet combustion. In this work, internal flow fields in the binary droplet combustion of two polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, n ≥ 1, abbreviated as PODEn), i.e., PODE2 and PODE4, are characterized using micro-particle image velocimetry (Micro-PIV). The buoyancy-driven upward vapor flow around the droplet is found to initiate two opposite radial flows in the droplet, which form two vortex cores near the surface, while the gravitational effect and Marangoni effect resulting from the content and temperature gradients in the binary droplets can induce disturbance to the two flows. The binary droplets have comparable spatially averaged flow velocities at the stable evaporation stage to those of pure droplets, which are around 3 mm/s. The velocity curves are more fluctuant and tend to slightly increase and reach the peak values at around 250 ms, and then decrease until droplet atomization. The flow velocities in the droplet interior are generally higher than those near the droplet surface, forming a parabolic velocity profile along the horizontal radial direction. The peak velocity first increases to 5–9 mm/s as the radial flow and vortex structure start to form and then decreases to around 3 mm/s until droplet atomization. The radial flow with a spatially averaged velocity of 3 mm/s can run around one lap during the stable evaporation stage, which implies that the convection-induced mass transfer is relatively weak, and consequently, the content gradient of the binary droplet is still mainly controlled by mass diffusion.

Effect of surfactants during drop formation in a microfluidic channel: a combined experimental and computational fluid dynamics approach

The effect of surfactants on the flow characteristics during rapid drop formation in a microchannel is investigated using high-speed imaging, micro-particle image velocimetry and numerical simulations; the latter are performed using a three- dimensional multiphase solver that accounts for the transport of soluble surfactants in the bulk and at the interface. Drops are generated in a flow-focusing microchannel, using silicone oil ( $4.6$  mPa s) as the continuous phase and a 52 % w/w glycerol solution as the dispersed phase. A non-ionic surfactant (Triton X-100) is dissolved in the dispersed phase at concentrations below and above the critical micelle concentration. Good agreement is found between experimental and numerical data for the drop size, drop formation time and circulation patterns. The results reveal strong circulation patterns in the forming drop in the absence of surfactants, whose intensity decreases with increasing surfactant concentration. The surfactant concentration profiles in the bulk and at the interface are shown for all stages of drop formation. The surfactant interfacial concentration is large at the front and the back of the forming drop, while the neck region is almost surfactant free. Marangoni stresses develop away from the neck, contributing to changes in the velocity profile inside the drop.

Onset of Visible Capillary Waves from High-Frequency Acoustic Excitation.

Remarkably, the interface of a fluid droplet will produce visible capillary waves when exposed to acoustic waves. For example, a small (∼1 μL) sessile droplet will oscillate at a low ∼102 Hz frequency when weakly driven by acoustic waves at ∼106 Hz frequency and beyond. We measured such a droplet's interfacial response to 6.6 MHz ultrasound to gain insight into the energy transfer mechanism that spans these vastly different time scales, using high-speed microscopic digital transmission holography, a unique method to capture three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that low-frequency capillary waves are driven into existence via a feedback mechanism between the acoustic radiation pressure and the evolving shape of the fluid interface. The acoustic pressure is distributed in the standing wave cavity of the droplet, and as the shape of the fluid interface changes in response to the distributed pressure present on the interface, the standing wave field also changes shape, feeding back to produce changes in the acoustic radiation pressure distribution in the cavity. A physical model explicitly based upon this proposed mechanism is provided, and simulations using it were verified against direct observations of both the microscale droplet interface dynamics from holography and internal pressure distributions using microparticle image velocimetry. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.

Microbial stir bars: Light-activated rotation of tethered bacterial cells to enhance mixing in stagnant fluids.

Microfluidics devices are gaining significant interest in biomedical applications. However, in a micron-scale device, reaction speed is often limited by the slow rate of diffusion of the reagents. Several active and passive micro-mixers have been fabricated to enhance mixing in microfluidic devices. Here, we demonstrate external control of mixing by rotating a rod-shaped bacterial cell. This rotation is driven by ion transit across the bacterial flagellar stator complex. We first measured the flow fields generated by rotating a single bacterial cell rotationally locked to rotate either clockwise (CW) or counterclockwise (CCW). Micro-particle image velocimetry (μPIV) and particle tracking velocimetry results showed that a bacterial cell of ∼ 2.75 μm long, rotating at 5.75 ± 0.39 Hz in a counterclockwise direction could generate distinct micro-vortices with circular flow fields with a mean velocity of 4.72 ± 1.67 μm/s and maximum velocity of 7.90 μm/s in aqueous solution. We verified our experimental data with a numerical simulation at matched flow conditions, which revealed vortices of similar dimensions and speed. We observed that the flow-field diminished with increasing z-height above the plane of the rotating cell. Lastly, we showed that we could activate and tune rotational mixing remotely using strains engineered with proteorhodopsin, where rotation could be activated by controlled external illumination using green laser light (561 nm).