We report the first measurements of the intrinsic strain fluctuations of living cells using a recentlydeveloped tracer correlation technique along with a theoretical framework for interpreting such data in heterogeneous media with non-thermal driving. The fluctuations' spatial and temporal correlations indicate that the cytoskeleton can be treated as a course-grained continuum with powerlaw rheology, driven by a spatially random stress tensor field. Combined with recent cell rheology results, our data imply that intracellular stress fluctuations have a nearly 1/ω 2 power spectrum, as expected for a continuum with a slowly evolving internal prestress.PACS numbers: 87.16. Ac, 87.15.Ya, 87.10.+e An accurate physical picture of the viscoelasticity and motion of the cytoskeleton is crucial for a complete understanding of processes such as intracellular transport [1], cell crawling [2], and mechano-chemical transduction [2]. Microrheology [3], based on the analysis of embedded tracer particle motion, has recently emerged as an experimental probe of cytoskeleton viscoelasticity and dynamics [4,5,6,7]. The viscoelastic properties of eucaryotic cells arise from an intricate network of protein filaments driven by specialized motor proteins and directional polymerization, that convert the chemical energy of adenosine triphosphate (ATP) to mechanical work and motion. A cell is thus a nonequilibrium soft material whose fluctuations are actively driven. Unlike the thermal fluctuations in an equilibrium material, the amplitude and spatial distribution of active fluctuations can be controlled via biochemical signaling pathways; perhaps allowing the cell to locally adjust its' mechanical properties to suit its' needs. Indeed, microscopic force generators play a central role in existing cell mechanics models such as the sol-gel [8], soft glassy rheology [4] and tensegrity [9] hypotheses.In this Letter, we extend a recently introduced method, termed two-point microrheology [10], and show that it can be used to characterize the activity of intracellular force generators by directly measuring a cell's intrinsic, random stress fluctuations. Our experimental data and theoretical framework show that a cell can be modelled as a coarse-grained viscoelastic continuum driven by a spatially random stress field having a 1/ω 2 power spectrum in our observable frequency range, 1 < ω < 60 rad/s.There are two distinct approaches to microrheology: the active approach measures the displacements of tracer particles induced by external forces and the passive approach measures fluctuations of particle positions in the absence of driving forces. The active approach provides a direct measure of the complex shear modulus µ(ω). In equilibrium systems the passive approach also measures µ(ω) because of the fluctuation-dissipation theorem (FDT) [11]. Literature results in cells using singleparticle versions of the two approaches yield shear moduli differing by orders of magnitude and exhibiting qualitatively different frequency dependencies [4,6]. These ...

[1] Galileo Near Infrared Mapping Spectrometer (NIMS) data of volcanic thermal emission are analyzed to determine the power output of a number of Ionian volcanoes, from which are calculated volumetric eruption rates. A two-temperature model is used to determine surface temperatures and power output. Portions of Prometheus and six other volcanoes are found to be in excess of 1100 K: areas of Prometheus are at temperatures close to 1500 K. These are minimum eruption temperatures and are consistent with basaltic silicate volcanism. For the 14 volcanoes in G1INNSPEC01, volumetric eruption rates (E) are constrained between 3 and 300 m 3 /s for basaltic and ultramafic compositions. Actual E values lie between these limits. The small crack fractions (hot area to warm area ratio) implied from these fits are indicative of a relatively quiescent eruption style, such as the emplacement of a pahoehoe or a-a flow field. The volumetric eruption rates derived from the NIMS data are greater than those observed in similar styles of volcanism on Earth. It is apparent that for similar eruption styles, the areal extent of activity is greater on Io than on Earth. Derived flow thicknesses range from 0.1 to 13 m.

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The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution....