38 research outputs found
A Predictive Model for Convective Flows Induced by Surface Reactivity Contrast
Concentration gradients in a fluid along a reactive surface due to contrast
in surface reactivity generate convective flows. These flows result from
contributions by electro and diffusio osmotic phenomena. In this study we have
analyzed reactive patterns that release and consume protons, analogous to
bimetallic catalytic conversion of peroxide. Here, we present a simple
analytical model that accurately predicts the induced potentials and consequent
velocities in such systems over a wide range of input parameters. Our model is
tested against direct numerical solutions to the coupled Poisson,
Nernst-Planck, and Navier-Stokes equations. Our analysis can be used to predict
enhancement of mass transport and the resulting impact on overall catalytic
conversion, and is also applicable to predicting the speed of catalytic
nanomotors
Universality in microdroplet nucleation during solvent exchange in Hele-Shaw like channels
Micro and nanodroplets have many important applications such as in drug
delivery, liquid-liquid extraction, nanomaterial synthesis and cosmetics. A
commonly used method to generate a large number of micro or nanodroplets in one
simple step is solvent exchange (also called nanoprecipitation), in which a
good solvent of the droplet phase is displaced by a poor one, generating an
oversaturation pulse that leads to droplet nucleation. Despite its crucial
importance, the droplet growth resulting from the oversaturation pulse in this
ternary system is still poorly understood. We experimentally and theoretically
study this growth in Hele-Shaw like channels by measuring the total volume of
the oil droplets that nucleates out of it. In order to prevent the
oversaturated oil from exiting the channel, we decorated some of the channels
with a porous region in the middle. Solvent exchange is performed with various
solution compositions, flow rates and channel geometries, and the measured
droplets volume is found to increase with the P\'eclet number with an
approximate effective power law . A theoretical model is
developed to account for this finding. With this model we can indeed explain
the scaling, including the prefactor, which can collapse
all data of the "porous" channels onto one universal curve, irrespective of
channel geometry and composition of the mixtures. Our work provides a
macroscopic approach to this bottom-up method of droplet generation and may
guide further studies on oversaturation and nucleation in ternary systems.Comment: Published in Journal of Fluid Mechanics. 16 pages, 6 figure
Evaporation-triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures
When placed on rough hydrophobic surfaces, water droplets of diameter larger
than a few millimeters can easily form pearls, as they are in the Cassie-Baxter
state with air pockets trapped underneath the droplet. Intriguingly, a natural
evaporating process can drive such a Fakir drop into a completely wetting
(Wenzel) state. Our microscopic observations with simultaneous side and bottom
views of evaporating droplets upon transparent hydrophobic microstructures
elucidate the water-filling dynamics and the mechanism of this
evaporation-triggered transition. For the present material the wetting
transition occurs when the water droplet size decreases to a few hundreds of
micrometers in radius. We present a general global energy argument which
estimates the interfacial energies depending on the drop size and can account
for the critical radius for the transition.Comment: 4 pages, 6 figure
Quantifying effective slip length over micropatterned hydrophobic surfaces
We employ micro-particle image velocimetry (-PIV) to investigate laminar
micro-flows in hydrophobic microstructured channels, in particular the slip
length. These microchannels consist of longitudinal micro-grooves, which can
trap air and prompt a shear-free boundary condition and thus slippage
enhancement. Our measurements reveal an increase of the slip length when the
width of the micro-grooves is enlarged. The result of the slip length is
smaller than the analytical prediction by Philip et al. [1] for an infinitely
large and textured channel comprised of alternating shear-free and no-slip
boundary conditions. The smaller slip length (as compared to the prediction)
can be attributed to the confinement of the microchannel and the bending of the
meniscus (liquid-gas interface). Our experimental studies suggest that the
curvature of the meniscus plays an important role in microflows over
hydrophobic micro-ridges.Comment: 8 page
Effect of axially varying sandpaper roughness on bubbly drag reduction in Taylor-Couette turbulence
We experimentally investigate the influence of alternating rough and smooth
walls on bubbly drag reduction (DR). We apply rough sandpaper bands of width
between and , and roughness height ,
around the smooth inner cylinder (IC) of the Twente Turbulent Taylor-Couette
facility. Between sandpaper bands, the IC is left uncovered over similar width
, resulting in alternating rough and smooth bands, a constant pattern in
axial direction. We measure the DR in water that originates from introducing
air bubbles to the fluid at (shear) Reynolds numbers ranging
from to . Results are compared to bubbly DR
measurements with a completely smooth IC and an IC that is completely covered
with sandpaper of the same roughness . The outer cylinder is left smooth for
all variations. Results are also compared to bubbly DR measurements where a
smooth outer cylinder is rotating in opposite direction to the smooth IC. This
counter rotation induces secondary flow structures that are very similar to
those observed when the IC is composed of alternating rough and smooth bands.
For the measurements with roughness, the bubbly DR is found to initially
increase more strongly with , before levelling off to reach a
value that no longer depends on . This is attributed to a more
even axial distribution of the air bubbles, resulting from the increased
turbulence intensity of the flow compared to flow over a completely smooth wall
at the same . The air bubbles are seen to accumulate at the
rough wall sections in the flow. Here, locally, the drag is largest and so the
drag reducing effect of the bubbles is felt strongest. Therefore, a larger
maximum value of bubbly DR is found for the alternating rough and smooth walls
compared to the completely rough wall
Spontaneous Breakdown of Superhydrophobicity
In some cases water droplets can completely wet micro-structured
superhydrophobic surfaces. The {\it dynamics} of this rapid process is analyzed
by ultra-high-speed imaging. Depending on the scales of the micro-structure,
the wetting fronts propagate smoothly and circularly or -- more interestingly
-- in a {\it stepwise} manner, leading to a growing {\it square-shaped} wetted
area: entering a new row perpendicular to the direction of front propagation
takes milliseconds, whereas once this has happened, the row itself fills in
microseconds ({\it ``zipping''})Comment: Accepted for publication in Physical Review Letter
Network Formation and Sieving Performance of Self-Assembling Hydrogels
Self-assembling hydrogels, consisting of aqueous solutions of poly(ethylene glycol)s end-capped with perfluorocarbon groups (Rf−PEGs), were studied for their electrophoretic sieving performance. These materials form physical gels, with the end groups aggregated in hydrophobic cores. The gels display high sieving performance, expressed as a large mobility dependence on DNA size, for short double-stranded DNA fragments even at relatively low polymer concentrations (∼3 wt %). This interesting characteristic can be attributed to the dense packing of interconnected micelles that build up the hydrogel network. The physically connected micelles act as a permanent network on the time scale of DNA migration over the distance between micelle cores. A mobility plateau was observed for intermediate DNA sizes that were probably too large to sieve through the network of interconnected micelles and yet too small to reptate. This plateau was followed by a reptation regime for larger DNA sizes, that has similar resolving characteristics to that observed for entangled linear PEO solutions
Hollow fiber ultrafiltration membranes with microstructured inner skin
Hollow fiber membranes with microstructured inner surfaces were fabricated from a PES/PVP blend using a spinneret with a microstructured needle. The effect of spinning parameters such as polymer dope flow rate, bore liquid flowrate, air gap and take-up speed on the microstructure and shape of the bore and its deformation was investigated. It was found that when a high bore liquid flowrate was used, the microstructure in the bore surface was destroyed. The bores were deformed to an oval shape when the fiber walls were thick. This was attributed to buckling of the fiber shell as a result of the coagulation and shrinkage of the outer surface. Fibers were also fabricated with a round-needled spinneret for comparison. The intrinsic pure water permeabilities (based on the actual bore surface areas) of fibers with structured and round bores were found to be similar. On the other hand, the structured fibers have larger pores in the skin layer. Smaller pores on the round fibers are considered to form when the inner surface coagulates and the skin layer is pulled inwards due to the shrinkage caused by phase separation. When the bore is structured, the wavy shape can damp this contraction effect resulting in larger pores. The skin layer thickness of the fibers was investigated using a colloidal filtration method. It was shown that fibers with microstructured bores which have mostly uniform skin layer thickness and reasonably narrow pore size distribution can be fabricate
Fouling behavior of microstructured hollow fiber membranes in submerged and aerated filtrations
The performance of microstructured hollow fiber membranes in submerged and aerated systems was investigated using colloidal silica as a model foulant. The microstructured fibers were compared to round fibers and to twisted microstructured fibers in flux-stepping experiments. The fouling resistances in the structured fibers were found to be higher than those of round fibers. This was attributed to stagnant zones in the grooves of the structured fibers. As the bubble sizes were larger than the size of the grooves of the structured fibers, it is possible that neither the bubbles nor the secondary flow caused by the bubbles can reach the bottom parts of the grooves. Twisting the structured fibers around their axes resulted in decreased fouling resistances. Large, cap-shaped bubbles and slugs were found to be the most effective in fouling removal, while small bubbles of sizes similar to the convolutions in the structured fiber did not cause an improvement in these fibers. Modules in a vertical orientation performed better than horizontal modules when coarse bubbling was used. For small bubbles, the difference between vertical and horizontal modules was not significant. When the structured and twisted fibers were compared to round fibers with respect to the permeate flowrate produced per fiber length instead of the actual flux through the convoluted membrane area, they showed lower fouling resistance than round fibers. This is because the enhancement in surface area is more than the increase in resistance caused by stagnant zones in the grooves of the structured fibers. From a practical point of view, although the microstructure does not promote further turbulence in submerged and aerated systems, it can still be possible to enhance productivity per module with the microstructured fibers due to their high surface area-to-volume rati