Physical mechanisms of ESCRT-III-driven cell division.

Living systems propagate by undergoing rounds of cell growth and division. Cell division is at heart a physical process that requires mechanical forces, usually exerted by assemblies of cytoskeletal polymers. Here we developed a physical model for the ESCRT-III-mediated division of archaeal cells, which despite their structural simplicity share machinery and evolutionary origins with eukaryotes. By comparing the dynamics of simulations with data collected from live cell imaging experiments, we propose that this branch of life uses a previously unidentified division mechanism. Active changes in the curvature of elastic cytoskeletal filaments can lead to filament perversions and supercoiling, to drive ring constriction and deform the overlying membrane. Abscission is then completed following filament disassembly. The model was also used to explore how different adenosine triphosphate (ATP)-driven processes that govern the way the structure of the filament is changed likely impact the robustness and symmetry of the resulting division. Comparisons between midcell constriction dynamics in simulations and experiments reveal a good agreement with the process when changes in curvature are implemented at random positions along the filament, supporting this as a possible mechanism of ESCRT-III-dependent division in this system. Beyond archaea, this study pinpoints a general mechanism of cytokinesis based on dynamic coupling between a coiling filament and the membrane

Numerical modeling of the nonlinear propagation of transient acoustic signals

6 pages.-- PACS nr.: 43.25.Cb.-- Communication presented at: Forum Acusticum Sevilla 2002 (Sevilla, Spain, 16-20 Sep 2002), comprising: 3rd European Congress on Acoustics; XXXIII Spanish Congress on Acoustics (TecniAcústica 2002); European and Japanese Symposium on Acoustics; 3rd Iberian Congress on Acoustics.-- Special issue of the journal Revista de Acústica, Vol. XXXIII, year 2002.A numerical study of high-amplitude transient signals propagating in an absorbing, homogeneous fluid is presented. The work is motivated in many applications where high intensity waves, which can not be described by linear laws, are involved (biomedical research, high power ultrasonics, etc.). Differential equations are written in lagrangian coordinates, and the full nonlinear equation is solved by means of a finite difference algorithm. Calculations are performed exclusively in the time domain, giving all the harmonic amplitudes by only one resolution step, and allowing the analysis of the evolution of the waveform for any original signal: gaussian, rectangular pulses, periodic excitation, etc. Numerical results are presented for waveform distortion and shock formation for plane transient and harmonic waves. Spatial and initial pulse shape dependences are specially analysed.Peer reviewe

Numerical simulation of nonlinear ultrasonic waves in bubbly liquid with nonhomogeneous bubble distribution

Ponencia presentada en el XIX Congreso Internacional de Acústica (ICA2007), Madrid, 2-7 Sep 2007.-- PACS: 43.25.Yw.In the framework of applications of power ultrasound in liquid, this paper deals with the analysis of nonlinear ultrasonic waves through a bubbly liquid. An open-field domain is assumed to contain a water and air-bubbles mixture. The presence of bubbles induces a strong nonlinear behaviour of the wave. A set of coupled differential equations is considered to model this phenomenon, and solved numerically. The effect of the bubble distribution on the nonlinearity of the acoustic field is analysed.This work has been suported by the research projects CM-URJC-CEF-091-4-M263, DPI2005-00894, and HA2005-0151.Peer reviewe

Ondas confinadas fuertemente no-lineales en fluidos termoviscosos: algunos experimentos numéricos

6 pp.-- PACS nr.: 43.25.Gf.-- Comunicación presentada en los siguientes congresos: II Congreso Iberoamericano de Acústica. XXXI Congreso Nacional de Acústica – TecniAcústica 2000. II Jornadas Iberoamericanas de Ultrasonidos. II Congreso Ibérico de Acústica. EAA Symposium on Architectural Acoustics (Madrid, 16-20 Octubre 2000).Publicado también en número especial de la Revista de Acústica, Vol. XXXI, año 2000.[EN] Strongly nonlinear standing waves in an one-dimensional rigid walled tube are numerically studied by using a finite difference algorithm. Calculations are performed exclusively in the time domain. No approximations about the Mach number are considered. Effects of absorption due to bulk attenuation are included and no approximations about its value are done. Different numerical experiments are shown including the formation of a strongly nonlinear continous standing wave as well as the behaviour of pulsed waves.[ES] El estudio del comportamiento de ondas de amplitud finita confinadas en resonadores acústicos es uno de los campos de investigación más prometedores en acústica nolineal, además de por su interés inherente también de cara a toda una gama de aplicaciones físicas y de ingeniería. Sin embargo la falta de una maquinaria matemática suficientemente desarrollada es un impedimento para la puesta a punto de tales aplicaciones. En este contexto, se presenta aquí un estudio numérico de ondas acústicas de gran amplitud confinadas en una cavidad cilíndrica. Las peculiaridades de este tipo de ondas, que son consecuencia fundamentalmente del comportamiento nolineal del fluido, se explican e ilustran mediante resultados de diferentes experimentos numéricos. Para ello se desarrolló un código de cálculo numérico, basado en un algoritmo de alto orden e implícito de diferencias finitas. El algoritmo numérico, en contraposición con otros que estudian el mismo problema físico y que se pueden encontrar en la literatura, resuelve la ecuación completamente nolineal, sin ningún tipo de aproximación en lo que se refiere al número de Mach y/o al valor de la atenuación, además de trabajar en el dominio del tiempo.Peer reviewe

Nonlinear ultrasonic waves in water-air mixtures

Communication presented at: Acoustics '08 (Paris, France, June 29-July 4, 2008), comprising: 5th Forum Acusticum, 155th ASA Meeting, 9e Congrès Français d'Acoustique, 7th European Conference on Noise Control (Euronoise), 9th European Conference on Underwater Acoustics (ECUA).-- Book of abstracts published in Vol. 94 Suppl.1: S1-S1020 (May/June 2008) of Acta Acustica united with Acustica: The Journal of the European Acoustics Association (ISSN 1610-1928).In this paper we present some features of nonlinear ultrasonic waves in water-air mixtures. This analysis is based on the coupling of the linear wave equation to the bubble equation in a volume formulation. The system is solved via the development of a numerical model (SMOW-BL code). The main restrictions of the model are: the nonlinear behaviour comes exclusively from the bubble vibration, all the bubbles have the same size, air inside the bubbles is adiabatic. Continuous waves at low ultrasonic frequency and wide band pulses propagation in an open domain are analysed. Results obtained by considering several bubbly layers in water are given. The existence of soliton in a water-air mixture is demonstrated. Some results corresponding to standing waves are also presented. A model which allows us to consider the self-generation of air bubbles in water, i.e., cavitation, is proposed.Peer reviewe

Numerical Modelling of some Problems in Nonlinear Acoustics

Communication presented at the 17th International Congress on Acoustics, Rome, 2-7 September 2001.Some recent developments in numerical nonlinear acoustics are presented. First a three dimensional perturbation approach based on the finite-element method is described. This procedure can predict the propagation of acoustic fields produced by sources of arbitrary geometry as well as the pressure distribution inside a three dimensional cavity including boundary layer absorption. Its main limitation is due to its range of validity limited to waves of finite but moderate amplitude. We then describe a second approach: a numerical model for nonlinear waves and weak shocks in thermoviscous fluids based in a time-domain finite-difference algorithm. This algorithm does not present any practical limitations about the amplitude of the wave but it is referred to one-dimensional problems. Some future trends are also commented.Peer reviewe

Numerical model for nonlinear standing waves and weak shocks in thermoviscous fluids

8 pages.-- PACS: 43.25.GfNonlinear standing waves in a one-dimensional tube are studied numerically by using a finite-difference algorithm. The numerical code models the acoustic field in resonators for homogeneous, thermoviscous fluids. Calculations are performed exclusively in the time domain, and all harmonic components are obtained by one resolution. The fully nonlinear differential equation is written in Lagrangian coordinates. It is solved without truncation. Effects of absorption are included. Displacement and pressure wave forms are calculated at different locations and results are shown for different excitation levels and tube lengths. Amplitude distributions along the resonator axis for every harmonic component are also evaluated. Simulations are performed for amplitudes ranging from linear to strongly nonlinear and weak shock. A very good concordance with classic experimental and analytical results is obtained.Peer reviewe

Experimental observation of nonlinear self-focusing in the cavitation field

Ponencia presentada en el XIX Congreso Internacional de Acústica (ICA2007), Madrid, 2-7 Sep 2007.-- PACS: 43.25.Yw.The nonlinear self-focusing of the pressure field with onset of cavitation is experimentally and theoretically analysed in the kHz range. Self-focusing in the cavitation field has already been reported above 500 kHz and it was attributed to the non-uniformity of bubble density. In this case, bubble density remains small, non-linearity is weak and focal distance remains much larger than the acoustic wavelength in the liquid.In this work, inertial cavitation field radiated by a 20 kHz sonotrode-type transducer is considered. A cone like bubble structure is established. This structure is very repetitive and selfconstructs in any container when a piston like emitter is used. The acoustic field is directly evaluated by using a BR&K. hydrophone and averaged pressure waveforms are analysed. The number of chosen samples is high enough to have stable measured pressure results (including nonlinear distortion). A self-focusing effect is observed with a focal distance comparable to the acoustic wavelength in the liquid. Models accounting for the high bubble density and strong non-linear effects are proposed. Theoretical predictions are compared to experiments and discussed.Peer reviewe

Numerical simulation of two-dimensional nonlinear standing acoustic waves

7 pages.-- PACS: 43.25GfIn this paper the behavior of strongly nonlinear waves in two-dimensional resonators filled with thermoviscous fluid is studied. For this purpose a set of differential equations, written in Lagrangian coordinates, is proposed and a time-domain numerical scheme is developed for solving them. Full nonlinear equations are derived from the conservation laws and state equation by assuming an irrotational fluid. Auxiliary conditions are written by considering a rigid-walled cavity, excitation at some points of the boundary, and rest at the outset. Finite differences are applied in the space and time domains, and lead to an implicit scheme. The numerical model solves the problem in terms of displacement vector field. The pressure field is then obtained from the displacement values. The algorithm allows us to analyze the evolution of the behavior of complex standing waves. The nonlinear characteristics of standing waves, well known in one-dimensional chambers, are now apparent in two-dimensional resonators by means of this new computational model. Some numerical experiments are carried out, a validation of the model is achieved, and results are given at a complex mode for which plane wave approximation is not appropriate. Several aspects of the nonlinear pressure field inside two-dimensional resonators are presented, such as harmonic distortion and nonlinear attenuation effects. In particular the quasi-standing character of such waves is detected and described. The effect of redistribution of rms pressure inside a two-dimensional cavity is commented.Peer reviewe