322 research outputs found
Non-linear mechanical, electrical and thermal phenomena in piezoelectric crystals
Mechanical, electrical and thermal phenomena occurring in piezoelectric
crystals were examined by non-linear approximation. For this purpose,
use was made of the thermodynamic function of state, which describes
an anisotropic body. Considered was the Gibbs function. The calculations
included strain tensor εij = f(σkl
, En, T), induction vector Dm =
f(σkl
, En, T) and entropy S = f(σkl
, En, T) as function of stress σkl
, field
strength En and temperature difference T. The equations obtained apply to
anisotropic piezoelectric bodies provided that the “forces” σkl
, En, T acting
on the crystal are known.Механічні, електричні та термічні явища у п’єзоелектричних кристалах вивчаються у нелінійному наближенні. З цією метою використано термодинамічний потенціал, який описує анізотропне тіло. Розглянуто потенціал Гіббса. Розрахунки охоплюють тензор деформації εij = f(σkl
, En, T), вектор індукції Dm = f(σkl
, En, T) та ентропію
S = f(σkl
, En, T) як функцію механічного напруження σkl
, величини
поля En і різниці температур T. Отримано рівняння, які описують
анізотропні п’єзоелектричні тіла, якщо відомі “сили” σkl
, En, T, що
діють на кристал
Phase separation and rotor self-assembly in active particle suspensions
Adding a non-adsorbing polymer to passive colloids induces an attraction
between the particles via the `depletion' mechanism. High enough polymer
concentrations lead to phase separation. We combine experiments, theory and
simulations to demonstrate that using active colloids (such as motile bacteria)
dramatically changes the physics of such mixtures. First, significantly
stronger inter-particle attraction is needed to cause phase separation.
Secondly, the finite size aggregates formed at lower inter-particle attraction
show unidirectional rotation. These micro-rotors demonstrate the self assembly
of functional structures using active particles. The angular speed of the
rotating clusters scales approximately as the inverse of their size, which may
be understood theoretically by assuming that the torques exerted by the
outermost bacteria in a cluster add up randomly. Our simulations suggest that
both the suppression of phase separation and the self assembly of rotors are
generic features of aggregating swimmers, and should therefore occur in a
variety of biological and synthetic active particle systems.Comment: Main text: 6 pages, 5 figures. Supplementary information: 5 pages, 4
figures. Supplementary movies available from
httP://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116334109/-/DCSupplementa
It’s all about information? The Following Behaviour of Professors and PhD Students on Twitter
In this paper we investigate the role of the academic status in the following behaviour of computer scientists on Twitter. Based on a uses and gratifications perspective, we focus on the activity of a Twitter account and the reciprocity of following relationships. We propose that the account activity addresses the users' information motive only, whereas the user's academic status relates to both the information motive and community development (as in peer networking or career planning). Variables were extracted from Twitter user data. We applied a biographical approach to correctly identify the academic status (professor versus PhD student). We calculated a MANOVA on the influence of the activity of the account and the academic status (on different groups of followers) to differentiate the influence of the information motive versus the motive for community development. Results suggest that for computer scientists Twitter is mainly an information network. However, we found significant effects in the sense of career planning, that is, the accounts of professors had even in the case of low activity a relatively high number of researcher followers -- both PhD followers as well as professor followers. Additionally, there was also some weak evidence for community development gratifications in the sense of peer-networking of professors. Overall, we conclude that the academic use of Twitter is not only about information, but also about career planning and networking
When are active Brownian particles and run-and-tumble particles equivalent? Consequences for motility-induced phase separation
Active Brownian particles (ABPs, such as self-phoretic colloids) swim at
fixed speed along a body-axis that rotates by slow angular
diffusion. Run-and-tumble particles (RTPs, such as motile bacteria) swim with
constant \u until a random tumble event suddenly decorrelates the
orientation. We show that when the motility parameters depend on density
but not on , the coarse-grained fluctuating hydrodynamics of
interacting ABPs and RTPs can be mapped onto each other and are thus strictly
equivalent. In both cases, a steeply enough decreasing causes phase
separation in dimensions , even when no attractive forces act between
the particles. This points to a generic role for motility-induced phase
separation in active matter. However, we show that the ABP/RTP equivalence does
not automatically extend to the more general case of \u-dependent motilities
Differential Dynamic Microscopy of Bacterial Motility
We demonstrate 'differential dynamic microscopy' (DDM) for the fast, high
throughput characterization of the dynamics of active particles. Specifically,
we characterize the swimming speed distribution and the fraction of motile
cells in suspensions of Escherichia coli bacteria. By averaging over ~10^4
cells, our results are highly accurate compared to conventional tracking. The
diffusivity of non-motile cells is enhanced by an amount proportional to the
concentration of motile cells.Comment: 4 pages, 4 figures. In this updated version we have added simulations
to support our interpretation, and changed the model for the swimming speed
probability distribution from log-normal to a Schulz distribution. Neither
modification significantly changes our conclusion
Enhanced gas-liquid mass transfer of an oscillatory constricted-tubular reactor
The mass transfer performance has been tested for gas-liquid flow in a new tubular reactor system, the oscillating mesotube (OMT), which features the oscillatory movement of fluid across a series of smooth constrictions located periodically along the vertical 4.4 mm internal diameter tube. The effect of the fluid oscillations (frequency,f, and center-to-peak amplitude, x(0), in the range of 0-20 s(-1) and 0-3 mm, respectively) on the overall volumetric mass transfer coefficient (k(L)a) has been tested by measuring the oxygen saturation levels with a fiber-optical microprobe (oxygen micro-optrode), and a mathematical model has been produced to describe the oxygen mass transport in the OMT. The oxygen mass transfer rates were about I order of magnitude higher (k(L)a values up to 0.16 s(-1)) than those values reported for gas-liquid contacting in a 50 mm internal diameter oscillatory flow reactor (OFR), for the same peak fluid oscillatory velocity, i.e., 2 pi fx(0). This represents remarkable oxygen transfer efficiencies, especially when considering the very low mean superficial gas velocity involved in this work (0.37 mm s(-1)). The narrower constrictions helped to increase the gas fraction (holdup) by reducing the rise velocity of the bubbles. However, the extent of radial mixing and the detachment of vortex rings from the surface of the periodic constrictions are actually the main causes of bubbles retention and effective gas-liquid contacting and are, thus, responsible for the enhancement of k(L)a in the OMT.N.R. thanks the Portuguese Foundation for Science and Technology (FCT) for financial support of his work (SFRH/BD/6954/2001)
Characterization and Control of the Run-and-Tumble Dynamics of {\it Escherichia Coli}
We characterize the full spatiotemporal gait of populations of swimming {\it
Escherichia coli} using renewal processes to analyze the measurements of
intermediate scattering functions. This allows us to demonstrate quantitatively
how the persistence length of an engineered strain can be controlled by a
chemical inducer and to report a controlled transition from perpetual tumbling
to smooth swimming. For wild-type {\it E.~coli}, we measure simultaneously the
microscopic motility parameters and the large-scale effective diffusivity,
hence quantitatively bridging for the first time small-scale directed swimming
and macroscopic diffusion
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