Crumpling of a stiff tethered membrane

first-principles numerical simulation model for crumpling of a stiff tethered membrane is introduced. In our model membranes, wrinkles, ridge formation, ridge collapse, as well as the initiation of stiffness divergence, are observed. The ratio of the amplitude and wave length of the wrinkles, and the scaling exponent of the stiffness divergence, are consistent with both theory and experiment. We observe that close to the stiffness divergence there appears a crossover beyond which the elastic behavior of a tethered membrane becomes similar to that of dry granular media. This suggests that ridge formation in membranes and force-chain network formation in granular packings are different manifestations of a single phenomenon.Comment: For full resolution figures, please send us an emai

Cationic DMPC/DMTAP Lipid Bilayers: Molecular Dynamics Study

Cationic lipid membranes are known to form compact complexes with DNA and to be effective as gene delivery agents both in vitro and in vivo. Here we employ molecular dynamics simulations for a detailed atomistic study of lipid bilayers consisting of a mixture of cationic dimyristoyltrimethylammonium propane (DMTAP) and zwitterionic dimyristoylphosphatidylcholine (DMPC). Our main objective is to examine how the composition of the bilayers affects their structural and electrostatic properties in the liquid-crystalline phase. By varying the mole fraction of DMTAP, we have found that the area per lipid has a pronounced non-monotonic dependence on the DMTAP concentration, with a minimum around the point of equimolar mixture. We show that this behavior has an electrostatic origin and is driven by the interplay between positively charged TAP headgroups and the zwitterionic PC heads. This interplay leads to considerable re-orientation of PC headgroups for an increasing DMTAP concentration, and gives rise to major changes in the electrostatic properties of the lipid bilayer, including a significant increase of total dipole potential across the bilayer and prominent changes in the ordering of water in the vicinity of the membrane. Moreover, chloride counter-ions are bound mostly to PC nitrogens implying stronger screening of PC heads by Cl ions compared to TAP head groups. The implications of these findings are briefly discussed

A new dimension to Turing patterns

It is well known that simple reaction-diffusion systems can display very rich pattern formation behavior. Here we have studied two examples of such systems in three dimensions. First we investigate the morphology and stability of a generic Turing system in three dimensions and then the well-known Gray-Scott model. In the latter case, we added a small number of morphogen sources in the system in order to study its robustness and the formation of connections between the sources. Our results raise the question of whether Turing patterning can produce an inductive signaling mechanism for neuronal growth.Comment: Movies available here at http://www.lce.hut.fi/research/polymer/turing.shtm

Lasten psykiatrisen perhekuntoutuksen kehittämishankkeen työaika- ja kustannusseuranta

Kela toteutti vuosina 2013–2016 kotona annettavaa 5–12-vuotiaiden lasten psykiatrista perhekuntoutusta koskevan kehittämishankkeen (Etä-LAKU). Osana kehittämishanketta tutkittiin kontrolloimattomassa seurantatutkimuksessa lasten perhekuntoutuksen aikana palveluntuottajien käyttämää työaikaa sekä työstä aiheutuneita henkilötyökustannuksia. Seurannassa kartoitettiin välitöntä ja välillistä työaikaa sekä kuntoutuksesta aiheutuneita kustannuksia kuntoutusprosessin eri vaiheissa. Kuntoutus toteutettiin yhden vuoden ja kuuden kuukauden (+ yhden optiovuoden) kestävänä monimuotoisena kuntoutusprosessina. Perhekuntoutusta annettiin mahdollisimman lähellä lapsen ja hänen perheensä arkielämää eli palveluntuottajat tulivat perheiden kotiin. Kuntoutus sisälsi aloituskäynnin, varsinaiset käynnit ja mahdollisesti vielä lisäkäyntejä. Varsinaisten käyntien aikana järjestettiin kaksi kuntoutusviikonloppua sekä seurantakäynti puolen vuoden päästä varsinaisten käyntien päättymisestä. Kehittämishankkeen kohderyhmänä olivat lapset (n = 78), joilla oli diagnosoitu neuropsykiatrinen häiriö, jonka lisäksi lapsella saattoi olla samanaikaisia psykiatrisia häiriöitä. Palveluntuottajien työaikaa kului enemmän välilliseen kuin välittömään työhön. Kotona sovellettavassa kuntoutusmallissa korostui palveluntuottajien matkustamisen osuus, joka vaihteli 29 prosentista 39 prosenttiin välillisestä työajasta. Kuntoutusprosessin kustannukset olivat kuntoutujakohtaisesti laskettuna palveluntuottajien omilla henkilötyökustannuksilla kalleimmillaan keskimäärin 13 971 euroa ja halvimmillaan keskimäärin 10 627 euroa. Kotona annettavasta psykiatrisesta perhekuntoutuksesta on hyvin vähän saatavilla kustannustietoa, joten tutkimusta kaivataan yhä edelleen

Molecular Dynamics Study of Charged Dendrimers in Salt-Free Solution: Effect of Counterions

Polyamidoamine (PAMAM) dendrimers, being protonated under physiological conditions, represent a promising class of nonviral, nano-sized vectors for drug and gene delivery. We performed extensive molecular dynamics simulations of a generic model dendrimer in a salt-free solution with dendrimer's terminal beads positively charged. Solvent molecules as well as counterions were explicitly included as interacting beads. We find that the size of the charged dendrimer depends non-monotonically on the strength of electrostatic interactions demonstrating a maximum when the Bjerrum length equals the diameter of a bead. Many other structural and dynamic characteristics of charged dendrimers are also found to follow this pattern. We address such a behavior to the interplay between repulsive interactions of the charged terminal beads and their attractive interactions with oppositely charged counterions. The former favors swelling at small Bjerrum lengths and the latter promotes counterion condensation. Thus, counterions can have a dramatic effect on the structure and dynamics of charged dendrimers and, under certain conditions, cannot be treated implicitly

Phase Diagram and Commensurate-Incommensurate Transitions in the Phase Field Crystal Model with an External Pinning Potential

We study the phase diagram and the commensurate-incommensurate transitions in a phase field model of a two-dimensional crystal lattice in the presence of an external pinning potential. The model allows for both elastic and plastic deformations and provides a continuum description of lattice systems, such as for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a mode expansion analysis is used to determine the ground states and the commensurate-incommensurate transitions in the model as a function of the strength of the pinning potential and the lattice mismatch parameter. Numerical minimization of the corresponding free energy shows good agreement with the analytical predictions and provides details on the topological defects in the transition region. We find that for small mismatch the transition is of first-order, and it remains so for the largest values of mismatch studied here. Our results are consistent with results of simulations for atomistic models of adsorbed overlayers

Spectral representation of the effective dielectric constant of graded composites

We generalize the Bergman-Milton spectral representation, originally derived for a two-component composite, to extract the spectral density function for the effective dielectric constant of a graded composite. This work has been motivated by a recent study of the optical absorption spectrum of a graded metallic film [Applied Physics Letters, 85, 94 (2004)] in which a broad surface-plasmon absorption band has been shown to be responsible for enhanced nonlinear optical response as well as an attractive figure of merit. It turns out that, unlike in the case of homogeneous constituent components, the characteristic function of a graded composite is a continuous function because of the continuous variation of the dielectric function within the constituent components. Analytic generalization to three dimensional graded composites is discussed, and numerical calculations of multilayered composites are given as a simple application.Comment: Physical Review E, submitted for publication