16 research outputs found
Dynamical fluctuations in mode locking experiments on vortices moving through mesoscopic channels
We have studied the flow properties of vortices driven through easy flow
mesoscopic channels by means of the mode locking (ML) technique. We observe a
ML jump with large voltage broadening in the real part of the rf-impedance.
Upon approaching the pure dc flow by reducing the rf amplitude, the ML jump is
smeared out via a divergence of the voltage width. This indicates a large
spread in internal frequencies and lack of temporal coherence in the dc-driven
state.Comment: 2 pages, 2 figures, contribution to M2S-HTSC 2003, Ri
Velocity oscillations in confined channel flows of concentrated colloidal suspensions
We study the pressure-driven flow of concentrated colloids confined in glass
micro-channels at the single particle level using fast confocal microscopy. For
channel to particle size ratios , the flow rate of the
suspended particles shows fluctuations. These turn into regular oscillations
for higher confinements (). We present evidence to link
these oscillations with the relative flow of solvent and particles (permeation)
and the effect of confinement on shear thickening.Comment: 4 pages, 6 figure
Dynamic mode locking in a driven colloidal system: experiments and theory
In this article we examine the dynamics of a colloidal particle driven by a
modulated force over a sinusoidal optical potential energy landscape. Coupling
between the competing frequencies of the modulated drive and that of particle
motion over the periodic landscape leads to synchronisation of particle motion
into discrete modes. This synchronisation manifests as steps in the average
particle velocity, with mode locked steps covering a range of average driving
velocities. The amplitude and frequency dependence of the steps are
considered, and compared to results from analytic theory, Langevin dynamics
simulations, and dynamic density functional theory. Furthermore, the critical
driving velocity is studied, and simulation used to extend the range of
conditions accessible in experiments alone. Finally, state diagrams from
experiment, simulation, and theory are used to show the extent of the
dynamically locked modes in two dimensions, as a function of both the
amplitude and frequency of the modulated drive
Tuning colloidal gels by shear
Using a powerful combination of experiments and simulations we demonstrate how the microstructure and its time evolution are linked with mechanical properties in a frustrated, out-of-equilibrium, particle gel under shear. An intermediate volume fraction colloid–polymer gel is used as a model system, allowing quantification of the interplay between interparticle attractions and shear forces. Rheometry, confocal microscopy and Brownian dynamics reveal that high shear rates, fully breaking the structure, lead after shear cessation to more homogeneous and stronger gels, whereas preshear at low rates creates largely heterogeneous weaker gels with reduced elasticity. We find that in comparison, thermal quenching cannot produce structural inhomogeneities under shear. We argue that external shear has strong implications on routes towards metastable equilibrium, and therefore gelation scenarios. Moreover, these results have strong implications for material design and industrial applications, such as mixing, processing and transport protocols coupled to the properties of the final material
Conching chocolate:A prototypical transition from frictionally jammed solid to flowable suspension with maximal solid content
The mixing of a powder of 10-50{\mu}m primary particles into a liquid to form
a dispersion with the highest possible solid content is a common industrial
operation. Building on recent advances in the rheology of such 'granular
dispersions', we study a paradigmatic example of such powder incorporation: the
conching of chocolate, in which a homogeneous, flowing suspension is prepared
from an inhomogeneous mixture of particulates, triglyceride oil and
dispersants. Studying the rheology of a simplified formulation, we find that
the input of mechanical energy and staged addition of surfactants combine to
effect a considerable shift in the jamming volume fraction of the system, thus
increasing the maximum flowable solid content. We discuss the possible
microscopic origins of this shift, and suggest that chocolate conching
exemplifies a ubiquitous class of powder-liquid mixing
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Linear and nonlinear microrheology of dense colloidal suspensions
The length and time scales accessible to optical tweezers make them an ideal tool for the examination of colloidal systems. Embedded high-refractive-index tracer particles in an index-matched hard sphere suspension provide 'handles' within the system to investigate the mechanical behaviour. Passive observations of the motion of a single probe particle give information about the linear response behaviour of the system, which can be linked to the macroscopic frequency-dependent viscous and elastic moduli of the suspension. Separate 'dragging' experiments allow observation of a sample's nonlinear response to an applied stress on a particle-by particle basis. Optical force measurements have given new data about the dynamics of phase transitions and particle interactions; an example in this study is the transition from liquid-like to solid-like behaviour, and the emergence of a yield stress and other effects attributable to nearest-neighbour caging effects. The forces needed to break such cages and the frequency of these cage breaking events are investigated in detail for systems close to the glass transition
Wavenumber-dependent dynamic light scattering optical coherence tomography measurements of collective and self-diffusion
We demonstrate wavenumber-dependent DLS-OCT measurements of collective and self-diffusion coefficients in concentrated silica suspensions across a broad q-range, utilizing a custom home-built OCT system. Depending on the sample polydispersity, either the collective or self-diffusion is measured. The measured collective-diffusion coefficient shows excellent agreement with hard-sphere theory and serves as an effective tool for accurately determining particle sizes. We employ the decoupling approximation for simultaneously measuring collective and self-diffusion coefficients, even in sufficiently monodisperse suspensions, using a high-speed Thorlabs OCT system. This enables particle size and volume fraction determination without the necessity of wavenumber-dependent measurements. We derive a relationship between the particle number-based polydispersity index and the ratio of self and collective mode amplitudes in the autocorrelation function and utilize it to measure the particle number-based polydispersity index. Notably, the polydispersity determined in this manner demonstrates improved sensitivity to smaller particle sizes compared to the standard intensity-based DLS cumulant analysis performed on dilute samples.ImPhys/Kalkman groupImPhys/Computational Imagin
Microscopic dynamics of synchronization in driven colloids.
Synchronization of coupled oscillators has been scrutinized for over three centuries, from Huygens' pendulum clocks to physiological rhythms. One such synchronization phenomenon, dynamic mode locking, occurs when naturally oscillating processes are driven by an externally imposed modulation. Typically only averaged or integrated properties are accessible, leaving underlying mechanisms unseen. Here, we visualize the microscopic dynamics underlying mode locking in a colloidal model system, by using particle trajectories to produce phase portraits. Furthermore, we use this approach to examine the enhancement of mode locking in a flexible chain of magnetically coupled particles, which we ascribe to breathing modes caused by mode-locked density waves. Finally, we demonstrate that an emergent density wave in a static colloidal chain mode locks as a quasi-particle, with microscopic dynamics analogous to those seen for a single particle. Our results indicate that understanding the intricate link between emergent behaviour and microscopic dynamics is key to controlling synchronization