601 research outputs found
Damping of liquid sloshing by foams: from everyday observations to liquid transport
We perform experiments on the sloshing dynamics of liquids in a rectangular
container submitted to an impulse. We show that when foam is placed on top of
the liquid the oscillations of the free interface are significantly damped. The
ability to reduce sloshing and associated splashing could find applications in
numerous industrial processes involving liquid transport.Comment: Accepted for publication in Journal of Visualizatio
Microfluidic In Situ Measurement of Poisson's Ratio of Hydrogels
Being able to precisely characterize the mechanical properties of soft
microparticles is essential for numerous situations from the understanding of
the flow of biological fluids to the development of soft micro-robots. Here we
present a simple measurement technique for the Poisson's ratio of soft
micron-sized hydrogels in the presence of a surrounding liquid. This methods
relies on the measurement of the deformation in two orthogonal directions of a
rectangular hydrogel slab compressed uni-axially inside a microfluidic channel.
Due to the in situ character of the method, the sample does not need to be
dried, allowing for the measurement of the mechanical properties of swollen
hydrogels. Using this method we determine the Poisson's ratio of hydrogel
particles composed of polyethylene glycol (PEG) and varying solvents fabricated
using a lithography technique. The results demonstrate with high precision the
dependence of the hydrogel compressibility on the solvent fraction and
character. The method, easy to implement, can be adapted for the measurement of
a variety of soft and biological materials
The recombinase protein is a torque sensitive molecular switch
How a nano-searcher finds its nano-target is a general problem in non-equilibrium statistical physics. It becomes vital when the searcher is a damaged DNA fragment trying to find its counterpart on the intact homologous chromosome. If the two copies are paired, that intact homologous sequence serves as a template to reconstitute the damaged DNA sequence, enabling the cell to survive without genetic mutations. To succeed, the search must stop only when the perfect homology is found. The biological process that ensures such a genomic integrity is called Homologous Recombination and is promoted by the Recombinase proteins. In this article, we use torque-sensitive magnetic tweezers to measure the free-energy landscape of the human Recombinase hRad51 protein assembled a DNA fragment. Based on our measurements we model the hRad51/DNA complex as an out-of-equilibrium two-state system and provide a thermodynamical description of Homologous Recombination. With this dynamical two-state model, we suggest a mechanism by which the recombinase proteins discriminate between homologous and a non-homologous sequences
Stress Clamp Experiments on Multicellular Tumor Spheroids
The precise role of the microenvironment on tumor growth is poorly
understood. Whereas the tumor is in constant competition with the surrounding
tissue, little is known about the mechanics of this interaction. Using a novel
experimental procedure, we study quantitatively the effect of an applied
mechanical stress on the long-term growth of a spheroid cell aggregate. We
observe that a stress of 10 kPa is sufficient to drastically reduce growth by
inhibition of cell proliferation mainly in the core of the spheroid. We compare
the results to a simple numerical model developed to describe the role of
mechanics in cancer progression.Comment: 5 pages, 4 figure
Beads, bubbles and drops in microchannels: stability of centered position and equilibrium velocity
Understand and predict the dynamics of dispersed micro-objects in
microfluidics is crucial in numerous natural, industrial and technological
situations. In this paper, we experimentally characterized the equilibrium
velocity and lateral position of various dispersed
micro-objects such as beads, bubbles and drops, in a cylindrical microchannel
over an unprecedent wide range of parameters. By systematically varying the
dimensionless object size (), the viscosity ratio (), the density ratio (), the
Reynolds number (), and the capillary number
(), we offer a general study exploring various
dynamics from the nonderformable viscous regime to the deformable
visco-inertio-capillary regime, thus enabling to highlight the sole and
combined roles of inertia and capillary effects on lateral migration. The
experiments are compared and well-agree with a steady 3D Navier-Stokes model
for incompressible two-phase fluids including both the effects of inertia and
possible interfacial deformations. This model enables to rationalize the
experiments and to provide an exhaustive parametric analysis on the influence
of the main parameters of the problem, mainly on two aspects: the stability of
the centered position and the velocity of the dispersed object. Interestingly,
we propose a useful correlation for the object velocity as functions of the
, and , obtained in the
limit, but actually valid for a larger range of values of and
in the linear regimes.Comment: 22 pages, 11 figures, submitted to Journal of Fluid Mechanic
Mechanical behavior of multi-cellular spheroids under osmotic compression
The internal and external mechanical environment plays an important role in
tumorogenesis. As a proxy of an avascular early state tumor, we use
multicellular spheroids, a composite material made of cells, extracellular
matrix and permeating fluid. We characterize its effective rheology at the
timescale of minutes to hours by compressing the aggregates with osmotic shocks
and modeling the experimental results with an active poroelastic material that
reproduces the stress and strain distributions in the aggregate. The model also
predicts how the emergent bulk modulus of the aggregate as well as the
hydraulic diffusion of the percolating interstitial fluid are modified by the
preexisting active stress within the aggregate. We further show that the value
of these two phenomenological parameters can be rationalized by considering
that, in our experimental context, the cells are effectively impermeable and
incompressible inclusions nested in a compressible and permeable matrix
Probing Rad51-DNA interactions by changing DNA twist
In eukaryotes, Rad51 protein is responsible for the recombinational repair of double-strand DNA breaks. Rad51 monomers cooperatively assemble on exonuclease-processed broken ends forming helical nucleo-protein filaments that can pair with homologous regions of sister chromatids. Homologous pairing allows the broken ends to be reunited in a complex but error-free repair process. Rad51 protein has ATPase activity but its role is poorly understood, as homologous pairing is independent of adenosine triphosphate (ATP) hydrolysis. Here we use magnetic tweezers and electron microscopy to investigate how changes of DNA twist affect the structure of Rad51-DNA complexes and how ATP hydrolysis participates in this process. We show that Rad51 protein can bind to double-stranded DNA in two different modes depending on the enforced DNA twist. The stretching mode is observed when DNA is unwound towards a helical repeat of 18.6 bp/turn, whereas a non-stretching mode is observed when DNA molecules are not permitted to change their native helical repeat. We also show that the two forms of complexes are interconvertible and that by enforcing changes of DNA twist one can induce transitions between the two forms. Our observations permit a better understanding of the role of ATP hydrolysis in Rad51-mediated homologous pairing and strand exchang
Dichloroacetate prevents cisplatin-induced nephrotoxicity without compromising cisplatin anticancer properties
Cisplatin is an effective anticancer drug; however, cisplatin use often leads to nephrotoxicity, which limits its clinical effectiveness. In this study, we determined the effect of dichloroacetate, a novel anticancer agent, in a mouse model of cisplatin-induced AKI. Pretreatment with dichloroacetate significantly attenuated the cisplatin-induced increase in BUN and serum creatinine levels, renal tubular apoptosis, and oxidative stress. Additionally, pretreatment with dichloroacetate accelerated tubular regeneration after cisplatin-induced renal damage. Whole transcriptome sequencing revealed that dichloroacetate prevented mitochondrial dysfunction and preserved the energy-generating capacity of the kidneys by preventing the cisplatin-induced downregulation of fatty acid and glucose oxidation, and of genes involved in the Krebs cycle and oxidative phosphorylation. Notably, dichloroacetate did not interfere with the anticancer activity of cisplatin in vivo. These data provide strong evidence that dichloroacetate preserves renal function when used in conjunction with cisplatin
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