Inertial Mixing Research Articles

Inertial mixing of acoustically levitated droplets for time‐lapse protein crystallography

AbstractVarying the chemical consistency of acoustically levitated droplets opens up an in situ study of chemical and biochemical reactions in small volumes. However, the optimization of the mixing time and the minimization of the positional instability induced by solution dispensing are necessary for practical applications such as the study of the transient state of macromolecules crystallography during the ligand binding processes. For this purpose, we study the inertial mixing in a configuration compatible with the room‐temperature crystallography using the acoustic levitation diffractometer, therein solution drops ejected at high velocity collide and coalesce with droplets dispensed on acoustically levitated and rotating polymer thin‐film sample holders. With the proposed method, we are able to achieve the mixing time of ∼0.1 s for sub‐micro and a few microliter droplets. The observed short mixing time is ascribed to the rapid penetration of the solution into the droplets and confirmed by a computational fluid dynamic simulation. The demonstrated accelerated solution mixing is tested in a pilot time‐lapse protein crystallography experiment using the acoustic levitation diffractometer. The results indicate the detection of transient ligand binding state within 2 s after the solution dispensing, suggesting the feasibility of the proposed method for studying slow biochemical processes.

What Made the Sustained Intensification of Tropical Cyclone Fani in the Bay of Bengal? An Investigation Using Coupled Atmosphere–Ocean Model

The extremely severe tropical cyclone Fani (25 April–5 May 2019) unusually sustained high intensity for a prolonged duration over the Bay of Bengal (BoB). A regional coupled atmosphere–ocean model was used to investigate the atmospheric and oceanic conditions and processes responsible for the sustained intensification of the tropical cyclone (TC) Fani. The coupled model simulated the track and intensification/weakening stages of the cyclone reasonably well. A reduction in sea surface temperature (by −2 °C) and an increase in sea surface salinity due to cyclone-induced upwelling and inertial mixing was noticed in both observations and model. The passage of TC Fani over two geostrophic mesoscale warm-core eddies along the cyclone track was found to supply the necessary energy for the intensification of TC Fani. The sea surface height anomaly and tropical cyclone heat potential (TCHP) were higher during TC Fani than other pre-monsoon cyclones in the BoB. The anomalous TCHP in the warm-core eddy zones (i.e., in excess of >160 kJ cm−2) maintained the warm surface temperature and high air–sea heat fluxes. The air–sea latent heat flux and atmospheric wind shear were favourable for the intensification of the cyclone. The atmospheric moist static energy enhanced up to 360 kJ kg−1 with a deep vertical extension in the atmospheric column supporting the further intensification of TC Fani. Therefore, the unusual oceanic TCHP associated with mesoscale eddies, higher latent heat flux, and enhanced moist static energy in the atmosphere contributed to the sustained intensification of TC Fani for a prolonged period in the BoB.

The effects of baffle configuration and number on inertial mixing in a curved serpentine micromixer: Experimental and numerical study

Recently, the application of micromixers in microfluidic systems including chemical and biological assays has been widely accomplished. Passive micromixers, benefitting from the low-cost and a less-complex fabrication process, rely solely on their geometry. In particular, Dean vortices generated in curved microchannels enhance the mixing performance through chaotic advection. To improve the mixing performance at relatively low Reynolds numbers (i.e. 1 ≤ Re ≤ 50), this study introduces baffles into the side walls of curved serpentine micromixers with curvature angles of 280°, which constantly agitate, stretch and fold the fluids streams. Six different baffle configurations were designed and the effects of geometry and the number of baffles were investigated both experimentally and numerically. According to the experimental results, while the maximum outlet mixing index of the micromixer with no baffles was 0.61, that of the micromixer with quasi-rectangular baffles was 0.98 at a low Reynolds number of 20, indicating the major contribution of the generated chaotic advection by baffles. Furthermore, numerical results, which were in good agreement with experimental results, shed more light onto the mechanisms involved in micromixing in terms of the flow behavior and mixing index.

An assessment of the impact of oceanic initial conditions on the interaction of upper ocean with the tropical cyclones in the Arabian Sea

ABSTRACT The Regional Ocean Modelling System (ROMS) is utilised to estimate the cyclone-induced mixing and cooling associated with two tropical cyclones, namely Phet and Nilofar over the Arabian Sea (AS). Numerical experiments were carried out with realistic and idealistic oceanic conditions to assess the impact of oceanic initial condition and stratification on the interaction of upper ocean with the tropical cyclones in the AS. The model simulated profiles of temperature and baroclinic kinetic energy are analysed to examine the post-cyclonic cooling and vertical distribution of inertial kinetic energy which interacts with the upper-ocean stratification. Following the cyclone-induced upwelling, the inertial mixing acts to deepen the mixed layer depth (MLD) up to 50 and 100 m from its initial values of 15 and 22 m for the Phet and Nilofar, respectively. The analysis suggests that the combined effect of higher maximum sustained winds and slower translational speed of Nilofar cyclone leads to the excessive cooling (by 1.5°C) of the sea surface as compared to Phet. A decrement in the magnitude of cooling is observed when oceanic initial stratification was altered. The available baroclinic velocity shear at different depths found to modulate the magnitude of upper-ocean cooling in different model experiments.

Sensitivity and binding kinetics of an ultra-sensitive chemiluminescent enzyme-linked immunosorbent assay at arrays of antibodies

We have characterized the sensitivity and kinetics of a multiplex immunoassay system based on detection of chemiluminescence (CL) at arrays of antibodies. This enzyme-linked immunosorbent assay (ELISA) was based on the spotting of different antibodies in a circular pattern at the bottom of a well of a microtiter plate. Sandwich immunocomplexes within each spot were labeled with horse radish peroxidase, and CL was generated locally to each spot in the array from turnover of luminol substrate. CL from the arrays across the plate was collected in single images; long exposure times were used to maximize sensitivity, and short exposure times were used to extend the dynamic range at higher signals. Image analysis was used to determine the intensity of light from each spot in the array, and intensity was converted to concentration of protein via comparison to a calibration curve. To determine the intrinsic sensitivity of the CL ELISA array, streptavidin horseradish peroxidase (SA-HRP) was captured on an array spotted with biotinylated detection antibodies. The limit of detection (LOD) of SA-HRP was 105 aM, or 3200 enzymes per 50 μL. A single-plex assay for prostate specific antigen (PSA) was developed that had an LOD of 79 aM when the microtiter plate was shaken orbitally, comparable to the most sensitive immunoassays reported to date. Normalization of CL signals in the PSA assay to signal per molecule of SA-HRP showed that the efficiency of the shaken assay was ~40%. When the plates were not shaken, the efficiency was ~4.5%, i.e., ~9-fold lower than when shaken. To better understand the theoretical basis of the sensitivity of these assays, we developed COMSOL numerical models of the binding kinetics at the array for plates that were shaken orbitally and those not shaken. Experimental data from the orbitally shaken PSA assay were best modeled by inertial mixing in a three-layer system that included a 8-μm-thick concentration boundary layer. Experimental data from the unshaken PSA assay were well modeled by diffusion-limited kinetics. A single-plex assay for IL-10 was developed with an LOD of 69 aM or 1.5 fg/mL, and used to measure this cytokine in plasma and serum of 10 healthy individuals. A 5-plex assay for IL-5, IL-6, IL-10, IL-22, and TNF-α was developed with LODs of 56 aM, 237 aM, 69 aM, 88 aM, and 373 aM, respectively. The assay was used to measure these 5 cytokines in the plasma and serum of the same individuals. The correlation in concentration of IL-10 measured in single-plex and multiplex assays was good (r2 = 0.89; bias = 14.5%). The factors that result in the high sensitivity of CL ELISA arrays—mostly high signal to noise ratio of extended chemiluminescent imaging—are discussed. This multiplex CL ELISA could be used for sensitive profiling of multiple proteins for in vitro diagnostics and biomarker detection in the development of therapeutics.

Rapid mixing of colliding picoliter liquid droplets delivered through-space from piezoelectric-actuated pipettes characterized by time-resolved fluorescence monitoring.

Rapid mixing of aqueous solutions is a crucial first step to study the kinetics of fast biochemical reactions with high temporal resolution. Remarkable progress toward this goal has been made through the development of advanced stopped-flow mixing techniques resulting in reduced dead times, and thereby extending reaction monitoring capabilities to numerous biochemical systems. Concurrently, piezoelectric actuators for through-space liquid droplet sample delivery have also been applied in several experimental systems, providing discrete picoliter sample volume delivery and precision sample deposition onto a surface, free of confinement within microfluidic devices, tubing, or other physical constraints. Here, we characterize the inertial mixing kinetics of two aqueous droplets (130 pl) produced by piezoelectric-actuated pipettes, following droplet collision in free space and deposition on a surface in a proof of principle experiment. A time-resolved fluorescence system was developed to monitor the mixing and fluorescence quenching of 5-carboxytetramethylrhodamine (5-Tamra) and N-Bromosuccinimide, which we show to occur in less than 10 ms. In this respect, this methodology is unique in that it offers millisecond mixing capabilities for very small quantities of discrete sample volumes. Furthermore, the use of discrete droplets for sample delivery and mixing in free space provides potential advantages, including the elimination of the requirement for a physical construction as with microfluidic systems, and thereby makes possible and extends the experimental capabilities of many systems.

How valid is Taylor dispersion formula in slugs?

How valid is Taylor dispersion formula in slugs?

Estimation of oceanic subsurface mixing under a severe cyclonic storm using a coupled atmosphere–ocean–wave model

Abstract. A coupled atmosphere–ocean–wave model was used to examine mixing in the upper-oceanic layers under the influence of a very severe cyclonic storm Phailin over the Bay of Bengal (BoB) during 10–14 October 2013. The coupled model was found to improve the sea surface temperature over the uncoupled model. Model simulations highlight the prominent role of cyclone-induced near-inertial oscillations in subsurface mixing up to the thermocline depth. The inertial mixing introduced by the cyclone played a central role in the deepening of the thermocline and mixed layer depth by 40 and 15 m, respectively. For the first time over the BoB, a detailed analysis of inertial oscillation kinetic energy generation, propagation, and dissipation was carried out using an atmosphere–ocean–wave coupled model during a cyclone. A quantitative estimate of kinetic energy in the oceanic water column, its propagation, and its dissipation mechanisms were explained using the coupled atmosphere–ocean–wave model. The large shear generated by the inertial oscillations was found to overcome the stratification and initiate mixing at the base of the mixed layer. Greater mixing was found at the depths where the eddy kinetic diffusivity was large. The baroclinic current, holding a larger fraction of kinetic energy than the barotropic current, weakened rapidly after the passage of the cyclone. The shear induced by inertial oscillations was found to decrease rapidly with increasing depth below the thermocline. The dampening of the mixing process below the thermocline was explained through the enhanced dissipation rate of turbulent kinetic energy upon approaching the thermocline layer. The wave–current interaction and nonlinear wave–wave interaction were found to affect the process of downward mixing and cause the dissipation of inertial oscillations.

Submerged jet shearing of visco-plastic sludge

We present a well-resolved numerical analysis of submerged jet mixing (sludge recirculation) of municipal sludge. Using an augmented Lagrangian algorithm, we accurately identify the position of true inactive unmixed regions. The analysis includes inertial and visco-plastic effects in laminar flow regimes. We find that inertial mixing flows are more effective in mobilizing sludge when they mix sludge that has a small yield stress. For sludge with a high yield stress, we offer other means to reduce or even fully eliminate the inactive unmixed regions, which rely on redesigning the shape of digester tank. In addition, we look at the effect of nozzle diameter and position on the efficiency of mixing.

Experimental Investigation of Inertial Mixing in Colliding Droplets

Achieving the increasingly fast mixing requirements posed by the chemical, biological, and life science community for confined microchannel droplet flows remains an engineering challenge. The viscous and surface tension forces that often dominate microflows undermine fast, efficient mixing. A novel mixing arrangement based on droplet collisions has been developed that significantly improves mixing rates by utilizing inertia to rapidly rearrange fluid contents. This article experimentally investigates inertial droplet mixing in micro-flows following high-speed droplet pair collisions. The technique utilizes a gaseous flow for liquid droplet generation and transport with collisions occurring in Y-junction microchannel geometries. Mixing rates are quantified using differential fluorescent optical diagnostics, custom image processing algorithms, and statistical analysis. Measured droplet mixing times are compared to the characteristic time scales for mass and viscous diffusion and bulk convective transport. Results show that mixing times are decreased as the droplet pair collision inertia is increased, indicating the potential benefit for inertial collision mixing.

The role of the inertia of cloud drops in the evolution of the spectra during drop growth by diffusion

It is shown that the inertia of small droplets leads to the formation of drop velocity flux divergence, with the maximum of the spatial spectrum at scales from 1 cm to 2.5 cm for different values of the dissipation rate. the space-length of correlation of the divergence field, calculated using the Batchelor model of isotropic and homogeneous turbulence, is also of centimetre scale. These results are interpreted in such a way that the drop inertia leads to the formation of the centimetre-scale structure of a cloud with ‘spots’ of enhanced and decreased drop concentration. Because of temporal and spatial changes of the turbulent flow structure, droplets appear alternately within the areas of positive or negative drop-flux velocity divergence and tend to leave the areas of the positive and enter the areas of negative drop-flux velocity divergence. Drop motions through the interface between two cloudy, or cloudy and clear-air, volumes lead to drop exchange between these volumes (inertial drop mixing). the characteristic time of this process was shown to be of the same order as the characteristic time-scales of molecular diffusion, droplet evaporation and gravitational sedimentation. Possible effects of the inertial mixing are illustrated using a simple model, in which a comparatively large air volume consists of many centimetre-scale volumes of different drop-flux divergence and ascent velocity within a cloud updraught. Drop exchange between the centimetre-scale volumes, together with the generation of supersaturation in the ascending volumes, leads to the evolution of droplet size spectra. the inertial mixing ‘itself’ leads to a weak droplet spectrum broadening in the case of uniform initial droplet concentration. the droplet spectrum broadening appears to be much more pronounced in cases of cloudy and clear-air volumes mixing, especially when fresh nucleation is assumed. It is shown that inertial mixing leads to the homogenization of the drop spectrum, i.e. to the formation of the same drop spectrum shape in all centimetre-scale volumes. Thus, inertial mixing may be important for the formation of local droplet spectra. At the same time, the inertial mixing may lead to significant fluctuations of drop concentration in these volumes.

The role of the inertia of cloud drops in the evolution of the spectra during drop growth by diffusion

AbstractIt is shown that the inertia of small droplets leads to the formation of drop velocity flux divergence, with the maximum of the spatial spectrum at scales from 1 cm to 2.5 cm for different values of the dissipation rate. the space‐length of correlation of the divergence field, calculated using the Batchelor model of isotropic and homogeneous turbulence, is also of centimetre scale. These results are interpreted in such a way that the drop inertia leads to the formation of the centimetre‐scale structure of a cloud with ‘spots’ of enhanced and decreased drop concentration. Because of temporal and spatial changes of the turbulent flow structure, droplets appear alternately within the areas of positive or negative drop‐flux velocity divergence and tend to leave the areas of the positive and enter the areas of negative drop‐flux velocity divergence. Drop motions through the interface between two cloudy, or cloudy and clear‐air, volumes lead to drop exchange between these volumes (inertial drop mixing). the characteristic time of this process was shown to be of the same order as the characteristic time‐scales of molecular diffusion, droplet evaporation and gravitational sedimentation. Possible effects of the inertial mixing are illustrated using a simple model, in which a comparatively large air volume consists of many centimetre‐scale volumes of different drop‐flux divergence and ascent velocity within a cloud updraught. Drop exchange between the centimetre‐scale volumes, together with the generation of supersaturation in the ascending volumes, leads to the evolution of droplet size spectra. the inertial mixing ‘itself’ leads to a weak droplet spectrum broadening in the case of uniform initial droplet concentration. the droplet spectrum broadening appears to be much more pronounced in cases of cloudy and clear‐air volumes mixing, especially when fresh nucleation is assumed. It is shown that inertial mixing leads to the homogenization of the drop spectrum, i.e. to the formation of the same drop spectrum shape in all centimetre‐scale volumes. Thus, inertial mixing may be important for the formation of local droplet spectra. At the same time, the inertial mixing may lead to significant fluctuations of drop concentration in these volumes.