Enrico Segre

Dynamics of a vesicle in general flow

An approach to quantitatively study vesicle dynamics as well as biologically-related micro-objects in a fluid flow, which is based on the combination of a dynamical trap and a control parameter, the ratio of the vorticity to the strain rate, is suggested. The flow is continuously varied between rotational, shearing, and elongational in a microfluidic 4-roll mill device, the dynamical trap, that allows scanning of the entire phase diagram of motions, i.e., tank-treading (TT), tumbling (TU), and trembling (TR), using a single vesicle even at λ = ηin/ηout = 1, where ηin and ηout are the viscosities of the inner and outer fluids. This cannot be achieved in pure shear flow, where the transition between TT and either TU or TR is attained only at λ>1. As a result, it is found that the vesicle dynamical states in a general are presented by the phase diagram in a space of only 2 dimensionless control parameters. The findings are in semiquantitative accord with the recent theory made for a quasi-spherical vesicle, although vesicles with large deviations from spherical shape were studied experimentally. The physics of TR is also uncovered.

Vesicle dynamics in time-dependent elongation flow: wrinkling instability.

We present experimental results on the relaxation dynamics of vesicles subjected to a timedependent elongation flow. We observed and characterized a new instability, which results in the formation of higher order modes of the vesicle shape (wrinkles), after a switch in the direction of the gradient of the velocity. This surprising generation of membrane wrinkles can be explained by the appearance of a negative surface tension during the vesicle deflation, due to compression in a sign-switching transient. Moreover, the formation of buds in the vesicle membrane has been observed in the vicinity of the dynamical transition point.

Elastic turbulence in von Karman swirling flow between two disks

We discuss the role of elastic stress in the statistical properties of elastic turbulence, realized by the flow of a polymer solution between two disks. The dynamics of the elastic stress are analogous to those of a small-scale fast dynamo in magnetohydrodynamics, and to those of the turbulent advection of a passive scalar in the Batchelor regime. Both systems are theoretically studied in the literature, and this analogy is exploited to explain the statistical properties, the flow structure, and the scaling observed experimentally. The following features of elastic turbulence are confirmed experimentally and presented in this paper: (i) The rms of the vorticity (and that of velocity gradients) saturates in the bulk of the elastic turbulent flow, leading to the saturation of the elastic stress. (ii) The rms of the velocity gradients (and thus the elastic stress) grows linearly with Wi in the boundary layer, near the driving disk. The rms of the velocity gradients in the boundary layer is one to two orders ...

Dynamics of interacting vesicles and rheology of vesicle suspension in shear flow

We studied the dynamics of isolated vesicles as well as vesicle interactions in semi-dilute vesicle suspensions subjected to a shear flow. We found that the long-range hydrodynamic interactions between vesicles give rise to strong fluctuations of vesicle shape and inclination angle, , though the functional dependence of and the transition path to tumbling motion is preserved. The dependence of the suspension viscosity on the viscosity ratio between inner and outer fluids, ?, was found to be non-monotonic and surprisingly growing with ? at the fixed outer fluid viscosity for ?<1, at odds with recent predictions made for a dilute suspension of non-interacting vesicles.

Wettability Stabilizes Fluid Invasion into Porous Media via Nonlocal, Cooperative Pore Filling.

We study the impact of the wetting properties on the immiscible displacement of a viscous fluid in disordered porous media. We present a novel pore-scale model that captures wettability and dynamic effects, including the spatiotemporal nonlocality associated with interface readjustments. Our simulations show that increasing the wettability of the invading fluid (the contact angle) promotes cooperative pore filling that stabilizes the invasion and that this effect is suppressed as the flow rate increases, due to viscous instabilities. We use scaling analysis to derive two dimensionless numbers that predict the mode of displacement. By elucidating the underlying mechanisms, we explain classical yet intriguing experimental observations. These insights could be used to improve technologies such as hydraulic fracturing, CO2 geosequestration, and microfluidics.

Bioluminescent response of individual dinoflagellate cells to hydrodynamic stress measured with millisecond resolution in a microfluidic device

SUMMARY Dinoflagellate bioluminescence serves as a model system for examining mechanosensing by suspended motile unicellular organisms. The response latency, i.e. the delay time between the mechanical stimulus and luminescent response, provides information about the mechanotransduction and signaling process, and must be accurately known for dinoflagellate bioluminescence to be used as a flow visualization tool. This study used a novel microfluidic device to measure the response latency of a large number of individual dinoflagellates with a resolution of a few milliseconds. Suspended cells of several dinoflagellate species approximately 35 μm in diameter were directed through a 200 μm deep channel to a barrier with a 15 μm clearance impassable to the cells. Bioluminescence was stimulated when cells encountered the barrier and experienced an abrupt increase in hydrodynamic drag, and was imaged using high numerical aperture optics and a high-speed low-light video system. The average response latency for Lingulodinium polyedrum strain HJ was 15 ms (N>300 cells) at the three highest flow rates tested, with a minimum latency of 12 ms. Cells produced multiple flashes with an interval as short as 5 ms between individual flashes, suggesting that repeat stimulation involved a subset of the entire intracellular signaling pathway. The mean response latency for the dinoflagellates Pyrodinium bahamense, Alexandrium monilatum and older and newer isolates of L. polyedrum ranged from 15 to 22 ms, similar to the latencies previously determined for larger dinoflagellates with different morphologies, possibly reflecting optimization of dinoflagellate bioluminescence as a rapid anti-predation behavior.

Fluctuations in two-dimensional reversibly damped turbulence

Gallavotti proposed an equivalence principle in hydrodynamics, which states that forced-damped fluids can be equally well represented by means of the Navier-Stokes equations and by means of time-reversible dynamical systems called GNS. In the GNS systems, the usual viscosity is replaced by a state-dependent dissipation term which fixes one global quantity. The principle states that the mean values of properly chosen observables are the same for both representations of the fluid. In the same paper, the chaotic hypothesis of Gallavotti and Cohen is applied to hydrodynamics, leading to the conjecture that entropy fluctuations in the GNS system verify a relation first observed in non-equilibrium molecular dynamics. We tested these ideas in the case of two-dimensional fluids. We examined the fluctuations of global quadratic quantities in the statistically stationary state of (a) the Navier-Stokes equations and (b) the GNS equations. Our results are consistent with the validity of the fluctuation relation, and of the equivalence principle, indicating possible extensions thereof. Moreover, in these results the difference between the Gallavotti-Cohen fluctuation theorem and the Evans-Searles identity is evident. AMS classification scheme numbers: 82C05, 76F20

Late states of incompressible 2D decaying vorticity fields

Two-dimensional decaying turbulent flow is known to approach apparently stable states after a long time evolution. A few theories and models have been so far proposed to account for this relaxation. In this paper, we compare results of numerical experiments with the predictions of these theories to assess their applicability. We study the long time decay of initially multilevel vorticity fields on the periodic box, and characterize the outcoming final states. Our final states do not match the predictions of the theories; a broader variety of dipole profiles, as well as nonstationary final states are found. The problem of the robustness of the relaxational state with respect to variations of the Reynolds number and different numerical resolution is addressed. The observed configurations also do not necessarily possess the maximal energy, in contrast to what is anticipated by some of the theories. We are led to conclude that the mixing of the vorticity is generally not ergodic, and that some metastable configurations may inhibit the attainment of an equilibrium state.

The possible association between exposure to air pollution and the risk for congenital malformations

BACKGROUND Over the last decade, there is growing evidence that exposure to air pollution may be associated with increased risk for congenital malformations. OBJECTIVES To evaluate the possible association between exposures to air pollution during pregnancy and congenital malformations among infants born following spontaneously conceived (SC) pregnancies and assisted reproductive technology (ART) pregnancies. METHODS This is an historical cohort study comprising 216,730 infants: 207,825 SC infants and 8905 ART conceived infants, during the periods 1997-2004. Air pollution data including sulfur dioxide (SO2), particulate matter <10 µm (PM10), nitrogen oxides (NOx) and ozone (O3) were obtained from air monitoring stations database for the study period. Using a geographic information system (GIS) and the Kriging procedure, exposure to air pollution during the first trimester and the entire pregnancy was assessed for each woman according to her residential location. Logistic regression models with generalized estimating equation (GEE) approach were used to evaluate the adjusted risk for congenital malformations. RESULTS In the study cohort increased concentrations of PM10 and NOx pollutants in the entire pregnancy were associated with slightly increased risk for congenital malformations: OR 1.06(95% CI, 1.01-1.11) for 10 µg/m(3) increase in PM10 and OR 1.03(95% CI, 1.01-1.04) for 10 ppb increase in NOx. Specific malformations were evident in the circulatory system (for PM10 and NOx exposure) and genital organs (for NOx exposure). SO2 and O3 pollutants were not significantly associated with increased risk for congenital malformations. In the ART group higher concentrations of SO2 and O3 in entire pregnancy were associated (although not significantly) with an increased risk for congenital malformations: OR 1.06(95% CI, 0.96-1.17) for 1 ppb increase in SO2 and OR 1.15(95% CI, 0.69-1.91) for 10 ppb increase in O3. CONCLUSIONS Exposure to higher levels of PM10 and NOx during pregnancy was associated with an increased risk for congenital malformations. Specific malformations were evident in the circulatory system and genital organs. Among ART pregnancies possible adverse association of SO2 and O3 exposure was also observed. Further studies are warranted, including more accurate exposure assessment and a larger sample size for ART pregnancies, in order to confirm these findings.

Validity of the Taylor hypothesis in a random spatially smooth flow

The validity of the Taylor frozen flow hypothesis in a chaotic flow of a dilute polymer solution in a regime of elastic turbulence is investigated experimentally. By accurate time-dependent measurements of the flow field we study the velocity coherence between pairs of points displaced both in time and space and quantify the degree of applicability of the Taylor hypothesis. Alternatively, the frozen flow assumption is assessed by comparison of the measured velocity structure functions with the ones derived by a frozen flow assumption. The breakdown of the Taylor hypothesis is further discussed in both the context of strong velocity fluctuations and long-range spatial correlations, which are the result of the flow smoothness and lack of scale separation. Different choices of the advection velocity are tested and discussed.