6,211 research outputs found
Synchronization of the Frenet-Serret linear system with a chaotic nonlinear system by feedback of states
A synchronization procedure of the generalized type in the sense of Rulkov et
al [Phys. Rev. E 51, 980 (1995)] is used to impose a nonlinear Malasoma chaotic
motion on the Frenet-Serret system of vectors in the differential geometry of
space curves. This could have applications to the mesoscopic motion of
biological filamentsComment: 12 pages, 7 figures, accepted at Int. J. Theor. Phy
Design and implementation of a compliant robot with force feedback and strategy planning software
Force-feedback robotics techniques are being developed for automated precision assembly and servicing of NASA space flight equipment. Design and implementation of a prototype robot which provides compliance and monitors forces is in progress. Computer software to specify assembly steps and makes force feedback adjustments during assembly are coded and tested for three generically different precision mating problems. A model program demonstrates that a suitably autonomous robot can plan its own strategy
Charge reversal of colloidal particles
A theory is presented for the effective charge of colloidal particles in
suspensions containing multivalent counterions. It is shown that if colloids
are sufficiently strongly charged, the number of condensed multivalent
counterion can exceed the bare colloidal charge leading to charge reversal.
Charge renormalization in suspensions with multivalent counterions depends on a
subtle interplay between the solvation energies of the multivalent counterions
in the bulk and near the colloidal surface. We find that the effective charge
is {\it not} a monotonically decreasing function of the multivalent salt
concentration. Furthermore, contrary to the previous theories, it is found that
except at very low concentrations, monovalent salt hinders the charge reversal.
This conclusion is in agreement with the recent experiments and simulations
Polyelectrolyte Multilayering on a Charged Planar Surface
The adsorption of highly \textit{oppositely} charged flexible
polyelectrolytes (PEs) on a charged planar substrate is investigated by means
of Monte Carlo (MC) simulations. We study in detail the equilibrium structure
of the first few PE layers. The influence of the chain length and of a (extra)
non-electrostatic short range attraction between the polycations and the
negatively charged substrate is considered. We show that the stability as well
as the microstructure of the PE layers are especially sensitive to the strength
of this latter interaction. Qualitative agreement is reached with some recent
experiments.Comment: 28 pages; 11 (main) Figs - Revtex4 - Higher resolution Figs can be
obtained upon request. To appear in Macromolecule
Age-related decline of de novo T cell responsiveness as a cause of COVID-19 severity
To the Editor,
So far, little attention has been paid to the link between immunosenescence and the dramatic mortality rate of coronavirus disease 2019 (COVID-19) in older age groups. Indeed, the number of cases of COVID-19, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is very low among children and teenagers, in contrast to the increased frequency in adults and the elderly, who are also more at risk of developing very serious symptoms and death (Guan et al. 2020; Wu and McGoogan 2020). As shown in Fig. 1, a similar epidemiological profile was observed during previous coronavirus (severe acute respiratory syndrome coronavirus 1, SARS-CoV-1, and Middle east respiratory syndrome coronavirus, MERS-CoV) outbreaks (Jia et al. 2009; Salamatbakhsh et al. 2019). Notably, the same trend was also noted during West Nile virus and, with some exceptions in very young children, Ebolavirus outbreaks (Bower et al. 2016; Hayes et al. 2005). Likely this phenomenon is multifactorial. For instance, in elderly individuals with severe COVID-19, associated comorbidities are much more prevalent (Guan et al. 2020). In addition, the progressive accumulation of senescent cells during life may play a role in the vulnerability of old people to COVID-19, resulting in reduced functionality of the organs, such as the lungs, and facilitating conditions for the development of fibrosis. Moreover, senescent cells can generate a pro-inflammatory environment, referred to as SASP (for senescence-associated secretory phenotype), which includes many inflammatory cytokines (e.g., interleukin-6) and contributes to the basal hyperinflammatory status characteristic of the old person. This hyperinflammatory status might influence the expression of ACE2, CD147, cyclophilins, CD26, and other CoV-associated molecules in human tissues, thus favoring viral entry (Radzikowska et al. 2020). It likely also constitutes an already unbalanced pro-inflammatory background, on which the development of an exacerbated inflammatory response and acute respiratory distress syndrome may be facilitated upon SARS-CoV-2 infection
Dynamics of Collapse of flexible Polyelectrolytes and Polyampholytes
We provide a theory for the dynamics of collapse of strongly charged
polyelectrolytes (PEs) and flexible polyampholytes (PAs) using Langevin
equation. After the initial stage, in which counterions condense onto PE, the
mechanism of approach to the globular state is similar for PE and PA. In both
instances, metastable pearl-necklace structures form in characteristic time
scale that is proportional to N^{4/5} where N is the number of monomers. The
late stage of collapse occurs by merger of clusters with the largest one
growing at the expense of smaller ones (Lifshitz- Slyozov mechanism). The time
scale for this process T_{COLL} N. Simulations are used to support the proposed
collapse mechanism for PA and PE.Comment: 14 pages, 2 figure
Cavitation pressure in liquid helium
Recent experiments have suggested that, at low enough temperature, the
homogeneous nucleation of bubbles occurs in liquid helium near the calculated
spinodal limit. This was done in pure superfluid helium 4 and in pure normal
liquid helium 3. However, in such experiments, where the negative pressure is
produced by focusing an acoustic wave in the bulk liquid, the local amplitude
of the instantaneous pressure or density is not directly measurable. In this
article, we present a series of measurements as a function of the static
pressure in the experimental cell. They allowed us to obtain an upper bound for
the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium
3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic
transducer characteristics, we also obtained a lower bound (at low temperature,
P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article
we thus present quantitative evidence that cavitation occurs at low temperature
near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium
4). Further information is also obtained on the comparison between the two
helium isotopes. We finally discuss the magnitude of nonlinear effects in the
focusing of a sound wave in liquid helium, where the pressure dependence of the
compressibility is large.Comment: 11 pages, 9 figure
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