14 research outputs found
Enhanced singular jet formation in oil-coated bubble bursting
Bubbles are ubiquitous in many natural and engineering processes, and bubble
bursting aerosols are of particular interest because of their critical role in
mass and momentum transfer across interfaces. All prior studies claim that
bursting of a millimeter-sized bare bubble at an aqueous surface produces jet
drops with a typical size of (100 \si{\micro\relax}m), much
larger than film drops of (1 \si{\micro\relax}m) from the
disintegration of a bubble cap. Here, we document the hitherto unknown
phenomenon that jet drops can be as small as a few microns when the bursting
bubble is coated by a thin oil layer. We provide evidence that the faster and
smaller jet drops result from the singular dynamics of the oil-coated cavity
collapse. The unique air-oil-water compound interface offers a distinct damping
mechanism to smooth out the precursor capillary waves during cavity collapse,
leading to a more efficient focusing of the dominant wave and thus allowing
singular jets over a much wider parameter space beyond that of a bare bubble.
We develop a theoretical explanation for the parameter limits of the singular
jet regime by considering the interplay among inertia, surface tension, and
viscous effects. As such contaminated bubbles are widely observed, the
previously unrecognized fast and small contaminant-laden jet drops may enhance
bubble-driven flux across the interface, contributing to the aerosolization and
airborne transmission of bulk substances
Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions
Merging of different streams in channel junctions represents a common mixing process that occurs in systems ranging from soda fountains and bathtub faucets to chemical plants and microfluidic devices. Here, we report a spontaneous trapping of colloidal particles in a merging flow junction when the merging streams have a salinity contrast. We show that the particle trapping is a consequence of nonequilibrium interactions between the particles, solutes, channel, and the freestream flow. A delicate balance of transport processes results in a stable near-wall vortex that traps the particles. We use three-dimensional particle visualization and numerical simulations to provide a rigorous understanding of the observed phenomenon. Such a trapping mechanism is unique from the well-known inertial trapping enabled by vortex breakdown [Proc. Natl. Acad. Sci. USA 111, 4770 (2014)], or the solute-mediated trapping enabled by diffusiophoresis [Phys..Rev. X 7, 041038 (2017)], as the current trapping is facilitated by both the solute and the inertial effects, suggesting a new mechanism for particle trapping in flow networks
Viscosity measurements of glycerol in a parallel-plate rheometer exposed to atmosphere
Glycerol is a hygroscopic fluid that spontaneously absorbs water vapor from
the atmosphere. For applications involving glycerol, care must be taken to
avoid exposure to humidity, since its viscosity decreases quickly as water is
absorbed. We report experimental measurements of the viscosity of glycerol in a
parallel-plate rheometer where the outer interface is exposed to atmosphere.
The measurements decrease with time as water is absorbed from the atmosphere
and transported throughout the glycerol via diffusion and advection. Measured
viscosities drop faster at higher relative humidities, confirming the role of
hygroscopicity on the transient viscosities. The rate of viscosity decrease
shows a non-monotonic relationship with the rheometer gap height. This behavior
is explained by considering the transition from diffusion-dominated transport
in the narrow gap regime to the large gap regime where transport is dominated
by inertia-driven secondary flows. Numerical simulations of the water
absorption and transport confirm this non-monotonic behavior. The experimental
viscosity measurements show unexpectedly fast decreases at very small gap
heights, violating the parallel-plate, axisymmetric model. We propose that this
drop-off may be due to misalignment in the rheometer that becomes
non-negligible for small gaps. Theoretical considerations show that secondary
flows in a misaligned rheometer dominate the typical secondary inertial flows
in parallel-plate rheometers at small gaps. Finally, simulations in a
misaligned parallel-plate system demonstrate the same sharp drop-off in
viscosity measurements at small gap heights. This modeling can be used to
estimate the gap height where misalignment effects dominate the transient
glycerol viscosity measurements.Comment: 26 pages, 17 figure
Coupling of vortex breakdown and stability in a swirling flow
Swirling flows are ubiquitous over a large range of length scales and applications including micron-scale microfluidic devices up to geophysical flows such as tornadoes. As the viscous dissipation, shear, and centrifugal stresses interact, such flows can often exhibit unexpected fluid dynamics. Here, we use microfluidic experiments and numerical simulations to study the flow in a vortex T-mixer: a T-shaped channel with staggered, offset inlets. The vortex T-mixer flow is characterized by a single dominant vortex, the stability of which is closely coupled to the appearance of vortex breakdown. Specifically, at a Reynolds number of Re≈90, a first vortex breakdown region appears in the steady-state solution, rendering the vortex pulsatively unstable. A second vortex breakdown region appears at Re≈120, which restabilizes the vortex. Finally, a third vortex breakdown region appears at Re≈180, which renders the vortex helically unstable. Thus, a counterintuitive flow regime exists for the vortex T-mixer in which increasing the Reynolds number has a stabilizing effect on the steady-state flow. The pulsatively unstable vortex evolves into a periodically pulsating state with a Strouhal number of St≈0.5, and the helically unstable vortex evolves into a helically oscillating state with St≈1.75. These transitions can be explained within the framework of linear hydrodynamic stability. In addition, the vortex T-mixer flow exhibits multistability; multiple flow states are stable over various ranges of Re, including a narrow range of tristability for 160≤Re≤170, in which the steady state, the pulsatile oscillation, and the helical oscillation are all stable. This study provides experimental and numerical evidence of the close coupling between vortex breakdown and flow stability, including the restabilization of the flow with increasing Reynolds number due to the appearance of a vortex breakdown region, which will provide new insights into how vortex breakdown can affect the stability of a swirling flow
Vortex-Breakdown-Induced Particle Capture in Branching Junctions
We show experimentally that a flow-induced, Reynolds number-dependent particle-capture mechanism in branching junctions can be enhanced or eliminated by varying the junction angle. In addition, numerical simulations are used to show that the features responsible for this capture have the signatures of classical vortex breakdown, including an approach flow aligned with the vortex axis and a pocket of subcriticality. We show how these recirculation regions originate and evolve and suggest a physical mechanism for their formation. Furthermore, comparing experiments and numerical simulations, the presence of vortex breakdown is found to be an excellent predictor of particle capture. These results inform the design of systems in which suspended particle accumulation can be eliminated or maximized
Bostonia: The Boston University Alumni Magazine. Volume 16
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions
Flow-Driven Rapid Vesicle Fusion via Vortex Trapping
Fusion
between suspended lipid vesicles is difficult to achieve
without membrane proteins or ions because the vesicles have extremely
low equilibrium membrane tension and high poration energy. Nonetheless,
vesicle fusion in the absence of mediators can also be achieved by
mechanical forcing that is strong enough to induce membrane poration.
Here, we employ a strong fluid shear stress to achieve vesicle fusion.
By utilizing a unique vortex formation phenomenon in branched channels
as a platform for capturing, stressing, and fusing the lipid vesicles,
we directly visualize using high-speed imaging the vesicle fusion
events, induced solely by shear, on the time scale of submilliseconds.
We show that a large vesicle with a size of up to ∼10 μm
can be achieved by the fusion of nanoscale vesicles. This technique
has the potential to be utilized as a fast and simple way to produce
giant unilamellar vesicles and to serve as a platform for visualizing
vesicle interactions and fusions in the presence of shear
Accumulation of Colloidal Particles in Flow Junctions Induced by Fluid Flow and Diffusiophoresis
The flow of solutions containing solutes and colloidal particles in porous media is widely found in systems including underground aquifers, hydraulic fractures, estuarine or coastal habitats, water filtration systems, etc. In such systems, solute gradients occur when there is a local change in the solute concentration. While the effects of solute gradients have been found to be important for many applications, we observe an unexpected colloidal behavior in porous media driven by the combination of solute gradients and the fluid flow. When two flows with different solute concentrations are in contact near a junction, a sharp solute gradient is formed at the interface, which may allow strong diffusiophoresis of the particles directed against the flow. Consequently, the particles accumulate near the pore entrance, rapidly approaching the packing limit. These colloidal dynamics have important implications for the clogging of a porous medium, where particles that are orders of magnitude smaller than the pore width can accumulate and block the pores within a short period of time. We also show that this effect can be exploited as a useful tool for preconcentrating biomolecules for rapid bioassays