46 research outputs found
Charge Transport Scalings in Turbulent Electroconvection
We describe a local-power law scaling theory for the mean dimensionless
electric current in turbulent electroconvection. The experimental system
consists of a weakly conducting, submicron thick liquid crystal film supported
in the annulus between concentric circular electrodes. It is driven into
electroconvection by an applied voltage between its inner and outer edges. At
sufficiently large voltage differences, the flow is unsteady and electric
charge is turbulently transported between the electrodes. Our theoretical
development, which closely parallels the Grossmann-Lohse model for turbulent
thermal convection, predicts the local-power law . and are dimensionless
numbers that are similar to the Rayleigh and Prandtl numbers of thermal
convection, respectively. The dimensionless function , which is
specified by the model, describes the dependence of on the aspect ratio
. We find that measurements of are consistent with the theoretical
model.Comment: 12 pages, 7 figures, Submitted to Phys. Rev. E. See also
http://www.physics.utoronto.ca/nonlinea
How micropatterns and air pressure affect splashing on surfaces
We experimentally investigate the splashing mechanism of a millimeter-sized
ethanol drop impinging on a structured solid surface, comprised of
micro-pillars, through side-view and top-view high speed imaging. By increasing
the impact velocity we can tune the impact outcome from a gentle deposition to
a violent splash, at which tiny droplets are emitted as the liquid sheet
spreads laterally. We measure the splashing threshold for different
micropatterns and find that the arrangement of the pillars significantly
affects the splashing outcome. In particular, directional splashing in
direction in which air flow through pattern is possible. Our top-view
observations of impact dynamics reveal that an trapped air is responsible for
the splashing. Indeed by lowering the pressure of the surrounding air we show
that we can suppress the splashing in the explored parameter regime.Comment: 7 pages, 9 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
Electrolytically Generated Nanobubbles on HOPG Surfaces
Electrolysis of water is employed to produce surface nanobubbles on highly
orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles
are formed when the HOPG surface acts as negative (positive) electrode.
Coverage and volume of the nanobubbles enhance with increasing voltage. The
yield of hydrogen nanobubbles is much larger than the yield of oxygen
nanobubbles. The growth of the individual nanobubbles during the electrolysis
process is recorded in time with the help of AFM measurements and correlated
with the total current. Both the size of the individual nanobubbles and the
total current saturate after typical 1 minute; then the nanobubbles are in a
dynamic equilibrium, meaning that they do not further grow, in spite of ongoing
gas production and nonzero current. The surface area of nanobubbles shows a
good correlation with the nanobubble volume growth rate, suggesting that either
the electrolytic gas emerges directly at the nanobubbles' surface, or it
emerges at the electrode's surface and then diffuses through the nanobubbles'
surface. Moreover, the experiments reveal that the time constants of the
current and the aspect ratio of nanobubbles are the same under all conditions.
Replacement of pure water by water containing a small amount of sodium chloride
(0.01 M) allows for larger currents, but qualitatively gives the same results.Comment: Langmuir, in pres
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
Localized states in sheared electroconvection
Electroconvection in a thin, sheared fluid film displays a rich sequence of
bifurcations between different flow states as the driving voltage is increased.
We present a numerical study of an annular film in which a radial potential
difference acts on induced surface charges to drive convection. The film is
also sheared by independently rotating the inner edge of the annulus. This
simulation models laboratory experiments on electroconvection in sheared
smectic liquid crystal films. The applied shear competes with the electrical
forces, resulting in oscillatory and strongly subcritical bifurcations between
localized vortex states close to onset. At higher forcing, the flow becomes
chaotic via a Ruelle-Takens-Newhouse scenario. The simulation allows flow
visualization not available in the physical experiments, and sheds light on
previously observed transitions in the current-voltage characteristics of
electroconvecting smectic films.Comment: To be published in EuroPhysics Letters, 6 pages, 6 figures: final
versio
The Zipping-wetting Dynamics at the Breakdown of Superhydrophobicity
Under some conditions water droplets can completely wet micro-structured superhydrophobic surfaces. The dynamics of this rapid process is investigated with ultra-high-speed imaging. Depending on the scales of the micro-structure, the wetting fronts propagate smoothly and circularly or – more interestingly – in a stepwise manner for a smaller periodicity of the microstructure. The latter phenomenon leads to a growing square-shaped wetted area: liquid laterally enters a new row on a slow timescale of milliseconds, once it happens the row then fills itself towards the sides in microseconds (“zipping”)