939 research outputs found
Avalanche of particles in evaporating coffee drops
The pioneering work of Deegan et al. [Nature 389, (1997)] showed how a drying
sessile droplet suspension of particles presents a maximum evaporating flux at
its contact line which drags liquid and particles creating the well known
coffee stain ring. In this Fluid Dynamics Video, measurements using micro
Particle Image Velocimetry and Particle Tracking clearly show an avalanche of
particles being dragged in the last moments, for vanishing contact angles and
droplet height. This explains the different characteristic packing of the
particles in the layers of the ring: the outer one resembles a crystalline
array, while the inner one looks more like a jammed granular fluid. Using the
basic hydrodynamic model used by Deegan et al. [Phys. Rev. E 62, (2000)] it
will be shown how the liquid radial velocity diverges as the droplet life comes
to an end, yielding a good comparison with the experimental data.Comment: This entry contains a Fluid Dynamics Video candidate for the Gallery
of Fluid Motion 2011 and a brief article with informatio
Order-to-disorder transition in ring-shaped colloidal stains
A colloidal dispersion droplet evaporating from a surface, such as a drying
coffee drop, leaves a distinct ring-shaped stain. Although this mechanism is
frequently used for particle self-assembly, the conditions for crystallization
have remained unclear. Our experiments with monodisperse colloidal particles
reveal a structural transition in the stain, from ordered crystals to
disordered packings. We show that this sharp transition originates from a
temporal singularity of the flow velocity inside the evaporating droplet at the
end of its life. When the deposition speed is low, particles have time to
arrange by Brownian motion, while at the end, high-speed particles are jammed
into a disordered phase.Comment: accepted for PR
Oscillations of a gas pocket on a liquid-covered solid surface
The dynamic response of a gas bubble entrapped in a cavity on the surface of
a submerged solid subject to an acoustic field is investigated in the linear
approximation. We derive semi-analytical expressions for the resonance
frequency, damping and interface shape of the bubble. For the liquid phase, we
consider two limit cases: potential flow and unsteady Stokes flow. The
oscillation frequency and interface shape are found to depend on two
dimensionless parameters: the ratio of the gas stiffness to the surface tension
stiffness, and the Ohnesorge number, representing the relative importance of
viscous forces. We perform a parametric study and show, among others, that an
increase in the gas pressure or a decrease in the surface tension leads to an
increase in the resonance frequency until an asymptotic value is reached
Human-centred design methods : developing scenarios for robot assisted play informed by user panels and field trials
Original article can be found at: http://www.sciencedirect.com/ Copyright ElsevierThis article describes the user-centred development of play scenarios for robot assisted play, as part of the multidisciplinary IROMEC1 project that develops a novel robotic toy for children with special needs. The project investigates how robotic toys can become social mediators, encouraging children with special needs to discover a range of play styles, from solitary to collaborative play (with peers, carers/teachers, parents, etc.). This article explains the developmental process of constructing relevant play scenarios for children with different special needs. Results are presented from consultation with panel of experts (therapists, teachers, parents) who advised on the play needs for the various target user groups and who helped investigate how robotic toys could be used as a play tool to assist in the childrenâs development. Examples from experimental investigations are provided which have informed the development of scenarios throughout the design process. We conclude by pointing out the potential benefit of this work to a variety of research projects and applications involving humanârobot interactions.Peer reviewe
Apparatus to control and visualize the impact of a high-energy laser pulse on a liquid target
We present an experimental apparatus to control and visualize the response of
a liquid target to a laser-induced vaporization. We use a millimeter-sized drop
as target and present two liquid-dye solutions that allow a variation of the
absorption coefficient of the laser light in the drop by seven orders of
magnitude. The excitation source is a Q-switched Nd:YAG laser at its
frequency-doubled wavelength emitting nanosecond pulses with energy densities
above the local vaporization threshold. The absorption of the laser energy
leads to a large-scale liquid motion at timescales that are separated by
several orders of magnitude, which we spatiotemporally resolve by a combination
of ultra-high-speed and stroboscopic high-resolution imaging in two orthogonal
views. Surprisingly, the large-scale liquid motion at upon laser impact is
completely controlled by the spatial energy distribution obtained by a precise
beam-shaping technique. The apparatus demonstrates the potential for accurate
and quantitative studies of laser-matter interactions.Comment: Submitted to Review of Scientific Instrument
Building micro-soccer-balls with evaporating colloidal fakir drops
Evaporation-driven particle self-assembly can be used to generate
three-dimensional microstructures. We present a new method to create these
colloidal microstructures, in which we can control the amount of particles and
their packing fraction. To this end, we evaporate colloidal dispersion droplets
on a special type of superhydrophobic micro-structured surface, on which the
droplet re- mains in Cassie-Baxter state during the entire evaporative process.
The remainders of the droplet consist of a massive spherical cluster of the
microspheres, with diameters ranging from a few tens up to several hundreds of
microns. We present scaling arguments to show how the final particle packing
fraction of these balls depends on the dynamics of the droplet evaporation.Comment: Manuscript Submitted to Physical Review Letters, 29th February 201
COVID-19 vaccine roll-out in South Africa: The added value of wastewater surveillance for SARS-CoV-2
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