18 research outputs found
Soap films as two-dimensional fluids: Diffusion and flow fields
We observe tracer particles diffusing in soap films to measure the
two-dimensional (2D) viscous properties of the films. We make soap films with a
variety of water-glycerol mixtures and of differing thicknesses. The
single-particle diffusivity relates closely to parameters of the film (such as
thickness ) for thin films, but the relation breaks down for thicker films.
Notably, the diffusivity is faster than expected for thicker films, with the
transition at using the tracer particle diameter . This
indicates a transition from purely 2D diffusion to diffusion that is more
three-dimensional. Additionally, we measure larger length scale flow fields
from correlated particle motions and find good agreement with what is expected
from theory of 2D fluids for all our films, thin and thick. We measure the
effective 2D viscosity of a soap film using single-particle diffusivity
measurements in thin films, and using the two-particle correlation measurements
in all films
Temporal and spatial heterogeneity in aging colloids: a mesoscopic model
A coarse-grained model of dense hard sphere colloids building on simple
notions of particle mobility and spatial coherence is presented and shown to
reproduce results of experiments and simulations for key quantities such as the
intermediate scattering function, the particle mean-square displacement and the
mobility correlation function. All results are explained by two
emerging and interrelated dynamical properties: i) a rate of intermittent
events, quakes, which decreases as the inverse of the system age t; ii) a
length scale characterizing correlated domains, which increases linearly in log
t. This leads to simple and accurate scaling forms expressed in terms of a
single scaling variable Finally, we propose a method to experimentally extract
the growing length scale of an aging colloid and suggest that a suitable
scaling of the probability density function of particle displacement can
experimentally reveal the rate of quakes.Comment: 5 pages 6 figure
Plot of interfacial viscosity from single particle diffusion measurements as a function of <i>h/d</i>.
<p>Filled circles denote particles of diameter 0.1 <i>μ</i>m, open circles denote particles of diameter 0.18 <i>μ</i>m and diamonds denote particles of diameter 0.5 <i>μ</i>m. The horizontal light shaded region represents <i>η</i><sub><i>int</i></sub> = 1.42±0.74 nPa⋅s⋅m based on the mean and standard deviation of the data for <i>h</i>/<i>d</i> < 5. The vertical dark shaded region represents the crossover from physical behavior at small <i>h</i>/<i>d</i> to unphysical behavior at <i>h</i>/<i>d</i> > 5.2±0.9. The horizontal error bars are due to uncertainties of <i>h</i>, and vertical error bars are due to uncertainties of <i>h</i> and <i>η</i><sub>2<i>D</i></sub> (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e004" target="_blank">Eq 4</a>).</p
Two particle correlations in a single soap film measurement as a function of particle separation <i>R</i>.
<p>Particles of diameter <i>d</i> = 0.18 <i>μm</i> were used, and soap film thickness was <i>h</i> = 0.46±0.04 <i>μm</i>. The solid lines are fits from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121981#pone.0121981.e006" target="_blank">Eq 6</a> with <i>A</i> = 1.09 <i>μ</i>m<sup>2</sup>/<i>s</i>, <i>B</i> = 0.12 <i>μ</i>m<sup>2</sup>/<i>s</i> and <i>L</i> = 81 <i>μ</i>m. The data are computed from all particle pairs and averaging over a wide range of lag times <i>τ</i>.</p
Cartoon depicting soap film of thickness <i>h</i>, with a single representative particle of diameter <i>d</i> < <i>h</i>.
<p>On both air-film interfaces, representative soap molecules are shown. As discussed in the text, we believe it is likely that particles with <i>d</i> < <i>h</i> sit in the interior of the film, but we cannot rule out the possibility that some particles are trapped at the air-film interface.</p