69 research outputs found
Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture
When a macroscopic concentration gradient is present across a binary mixture,
long-ranged non-equilibrium concentration fluctuations (NCF) appear as a
consequence of the coupling between the gradient and spontaneous equilibrium
velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF)
may be also observed when the mixture is close to a critical point. Here we
study the interplay between NCF and critical ECF in a near critical mixture
aniline/cyclohexane in the presence of a vertical concentration gradient. To
this aim, we exploit a commercial optical microscope and a simple, custom-made,
temperature-controlled cell to obtain simultaneous static and dynamic
scattering information on the fluctuations. We first characterise the critical
ECF at fixed temperature above the upper critical solution temperature
, in the wide temperature range C. In this
range, we observe the expected critical scaling behaviour for both the
scattering intensity and the mass diffusion coefficient and we determine the
critical exponents , and , which are found in agreement
with the 3D Ising values. We then study the system in the two-phase region
(). In particular, we characterise the interplay between ECF and NCF
when the mixture, initially at a temperature , is rapidly brought to a
temperature . During the transient, a vertical diffusive mass flux
is present that causes the onset of NCF, whose amplitude vanishes with time, as
the flux goes to zero. We also study the time dependence of the equilibrium
scattering intensity , of the crossover wave-vector and of the
diffusion coefficient during diffusion and find that all these quantities
exhibit an exponential relaxation enslaved to the diffusive kinetics.Comment: 11 pages, 4 figure
Dark Field Differential Dynamic Microscopy enables the accurate characterization of the roto-translational dynamics of bacteria and colloidal clusters
Micro- and nanoscale objects with anisotropic shape are key components of a
variety of biological systems and inert complex materials, and represent
fundamental building blocks of novel self-assembly strategies. The time scale
of their thermal motion is set by their translational and rotational diffusion
coefficients, whose measurement may become difficult for relatively large
particles with small optical contrast. Here we show that Dark Field
Differential Dynamic Microscopy is the ideal tool for probing the
roto-translational Brownian motion of shape anisotropic particles. We
demonstrate our approach by successful application to aqueous dispersions of
non-motile bacteria and of colloidal aggregates of spherical particles
Dynamic scaling for the growth of non-equilibrium fluctuations during thermophoretic diffusion in microgravity
Diffusion processes are widespread in biological and chemical systems, where
they play a fundamental role in the exchange of substances at the cellular
level and in determining the rate of chemical reactions. Recently, the
classical picture that portrays diffusion as random uncorrelated motion of
molecules has been revised, when it was shown that giant non-equilibrium
fluctuations develop during diffusion processes. Under microgravity conditions
and at steady-state, non-equilibrium fluctuations exhibit scale invariance and
their size is only limited by the boundaries of the system. In this work, we
investigate the onset of non-equilibrium concentration fluctuations induced by
thermophoretic diffusion in microgravity, a regime not accessible to analytical
calculations but of great relevance for the understanding of several natural
and technological processes. A combination of state of the art simulations and
experiments allows us to attain a fully quantitative description of the
development of fluctuations during transient diffusion in microgravity. Both
experiments and simulations show that during the onset the fluctuations exhibit
scale invariance at large wave vectors. In a broader range of wave vectors
simulations predict a spinodal-like growth of fluctuations, where the amplitude
and length-scale of the dominant mode are determined by the thickness of the
diffuse layer.Comment: To appear in Scientific Report
Correcting artifacts from finite image size in Differential Dynamic Microscopy
Differential Dynamic Microscopy (DDM) analyzes traditional real-space
microscope images to extract information on sample dynamics in a way akin to
light scattering, by decomposing each image in a sequence into Fourier modes,
and evaluating their time correlation properties. DDM has been applied in a
number of soft-matter and colloidal systems. However, objects observed to move
out of the microscope's captured field of view, intersecting the edges of the
acquired images, can introduce spurious but significant errors in the
subsequent analysis. Here we show that application of a spatial windowing
filter to images in a sequence before they enter the standard DDM analysis can
reduce these artifacts substantially. Moreover, windowing can increase
significantly the accessible range of wave vectors probed by DDM, and may
further yield unexpected information, such as the size polydispersity of a
colloidal suspension
Differential dynamic microscopy microrheology of soft materials: A tracking-free determination of the frequency-dependent loss and storage moduli
Particle-tracking microrheology (PT-μr) exploits the thermal motion of embedded particles to probe the local mechanical properties of soft materials. Despite its appealing conceptual simplicity, PT-μr requires calibration procedures and operating assumptions that constitute a practical barrier to its wider application. Here we demonstrate differential dynamic microscopy microrheology (DDM-μr), a tracking-free approach based on the multiscale, temporal correlation study of the image intensity fluctuations that are observed in microscopy experiments as a consequence of the translational and rotational motion of the tracers. We show that the mechanical moduli of an arbitrary sample are determined correctly over a wide frequency range provided that the standard DDM analysis is reinforced with an iterative, self-consistent procedure that fully exploits the multiscale information made available by DDM. Our approach to DDM-μr does not require any prior calibration, is in agreement with both traditional rheology and diffusing wave spectroscopy microrheology, and works in conditions where PT-μr fails, providing thus an operationally simple, calibration-free probe of soft materials
Giant fluctuations and structural effects in a flocking epithelium
We thank S Henkes for useful discussions. FGia and RC acknowledge funding from the Italian Ministry of University and Scientific Research (MIUR) under the program Futuro in Ricerca—Project ANISOFT (RBFR125H0M) and from Regione Lombardia and CARIPLO foundation under the joint action Avviso congiunto per l'incremento dell'attrattivitá del sistema ricerca lombardo e della competitivitá dei ricercatori candidati su strumenti ERC—Project 2016-0998. CM, SC and GS acknowledge funding from Associazione Italiana per la Ricerca sul Cancro (AIRC 10168 and 18621), MIUR, the Italian Ministry of Health, Ricerca Finalizzata (RF0235844), Worldwide Cancer Research (AICR-14-0335), and the European Research Council (Advanced-ERC-268836). CM was also supported by Fondazione Umberto Veronesi and SC by an AIRC fellowship. FGin acknowledges support from the Marie Curie Career Integration Grant (CIG) PCIG13-GA-2013-618399, and wish to thank the University of Milan and LibrOsteria for their hospitality while this work was underway.Peer reviewedPostprin
Protein sizing with Differential Dynamic Microscopy
Introduced more than fifty years ago, dynamic light scattering is routinely
used to determine the size distribution of colloidal suspensions, as well as of
macromolecules in solution, such as proteins, nucleic acids, and their
complexes. More recently, differential dynamic microscopy has been proposed as
a way to perform dynamic light scattering experiments with a microscope, with
much less stringent constraints in terms of cleanliness of the optical
surfaces, but a potentially lower sensitivity due to the use of camera-based
detectors. In this work, we push bright-field differential dynamic microscopy
beyond known limits and show it to be sufficiently sensitive to size small
macromolecules in diluted solutions. By considering solutions of three
different proteins (Bovine Serum Albumin, Lysozyme, and Pepsin), we accurately
determine the diffusion coefficient and hydrodynamic radius of both single
proteins and small protein aggregates down to concentrations of a few
milligrams per milliliter. In addition, we present preliminary results showing
unexplored potential for the determination of virial coefficients. Our results
are in excellent agreement with the ones obtained in parallel with a
state-of-the-art commercial dynamic light scattering setup, showing that
differential dynamic microscopy represents a valuable alternative for rapid,
label-free protein sizing with an optical microscope
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