32 research outputs found
Diffusion of self-propelled particles in complex media
The diffusion of active microscopic organisms in complex environments plays
an important role in a wide range of biological phenomena from cell colony
growth to single organism transport. Here, we investigate theoretically and
computationally the diffusion of a self-propelled particle (the organism)
embedded in a complex medium comprised of a collection of non-motile solid
particles that mimic soil or other cells. Under such conditions we find that
the rotational relaxation time of the swimming direction depends on the
swimming velocity and is drastically reduced compared to a pure Newtonian
fluid. This leads to a dramatic increase (of several orders of magnitude) in
the effective rotational diffusion coefficient of the self-propelled particles,
which can lead to "self-trapping" of the active particles in such complex
media. An analytical model is put forward that quantitatively captures the
computational results. Our work sheds light on the role that the environment
plays in the behavior of active systems and can be generalized in a
straightforward fashion to understand other synthetic and biological active
systems in heterogenous environments
Aggregation dynamics of active rotating particles in dense passive media
Active matter systems are able to exhibit emergent non-equilibrium behavior due to activity-induced effective interactions between the active particles. Here we study the aggregation and dynamical behavior of active rotating particles, spinners, embedded in 2D passive colloidal monolayers. Using both experiments and simulations we observe aggregation of active particles or spinners whose behavior resembles classical 2D Cahn–Hilliard coarsening. The aggregation behavior and spinner attraction depend on the mechanical properties of the passive monolayer and the activity of spinners. Spinner aggregation only occurs when the passive monolayer behaves elastically and when the spinner activity exceeds a minimum activity threshold. Interestingly, for the spinner concentrations investigated here, the spinner concentration does not seem to change the dynamics of the aggregation behavior. There is a characteristic cluster size which maximizes spinner aggregation by minimizing the drag through the passive monolayer and maximizing the stress applied on the passive medium. We also show a ternary mixture of passive particles and co-rotating and counter-rotating spinners that aggregate into clusters of co and counter-rotating spinners respectivelyThis work was supported by Department of Energy BES award #ER46919 (theoretical and simulation work) and the Chang Family (experimental work)
Aqueous two-phase systems within selectively permeable vesicles
An aqueous two-phase system (ATPS) encapsulated within a
vesicle organizes the vesicle core as two coexisting phases that partition
encapsulated solutes. Here, we use microfluidic technologies to produce
vesicles that efficiently encapsulate mixtures of macromolecules, providing a
versatile platform to determine the phase behavior of ATPSs. Moreover, we
use compartmentalized vesicles to investigate how membrane permeability
affects the dynamics of the encapsulated ATPS. Designing a membrane
selectively permeable to one of the components of the ATPS, we show that
out-of-equilibrium phase separations formed by a rapid outflow of water can
be spontaneously reversed by a slower outflow of the permeating component across the vesicle membrane. This dynamics may be
exploited advantageously by cells to separate and connect metabolic and signaling routes within their nucleoplasm or cytoplasm
depending on external conditionsThe authors acknowledge financial support from the Spanish
Ministry of Science and Innovation (MCI) through the María
de Maeztu Programme for Units of Excellence in R&D
(CEX2018-000805-M). L.R.A. and J.L.A. also acknowledge
financial support from MCI through the Ramón y Cajal
Programme (RYC2018-025575-I and RYC2019-028189-I;
MCI/AEI/FSE, UE
The phase diagram of water from quantum simulations
The phase diagram of water has been calculated for the TIP4PQ/2005 model, an
empirical rigid non-polarisable model. The path integral Monte Carlo technique
was used, permitting the incorporation of nuclear quantum effects. The
coexistence lines were traced out using the Gibbs-Duhem integration method,
once having calculated the free energies of the liquid and solid phases in the
quantum limit, which were obtained via thermodynamic integration from the
classical value by scaling the mass of the water molecule. The resulting phase
diagram is qualitatively correct, being displaced to lower temperatures by
15-20K. It is found that the influence of nuclear quantum effects are
correlated to the tetrahedral order parameter.Comment: 10 pages, 6 figures, 1 tabl
Anomalies in water as obtained from computer simulations of the TIP4P/2005 model: density maxima, and density, isothermal compressibility and heat capacity minima
The so-called thermodynamic anomalies of water form an integral part of the
peculiar behaviour of this both important and ubiquitous molecule. In this
paper our aim is to establish whether the recently proposed TIP4P/2005 model is
capable of reproducing a number of these anomalies. Using molecular dynamics
simulations we investigate both the maximum in density and the minimum in the
isothermal compressibility along a number of isobars. It is shown that the
model correctly describes the decrease in the temperature of the density
maximum with increasing pressure. At atmospheric pressure the model exhibits an
additional minimum in density at a temperature of about 200K, in good agreement
with recent experimental work on super-cooled confined water. The model also
presents a minimum in the isothermal compressibility close to 310K. We have
also investigated the atmospheric pressure isobar for three other water models;
the SPC/E and TIP4P models also present a minimum in the isothermal
compressibility, although at a considerably lower temperature than the
experimental one. For the temperature range considered no such minimum is found
for the TIP5P model.Comment: 23 pages, 8 figure
How rotating ATP synthases can modulate membrane structure
F1Fo-ATP synthase (ATP synthase) is a central membrane protein that synthetizes most of the ATP in the cell through a rotational movement driven by a proton gradient across the hosting membrane. In mitochondria, ATP synthases can form dimers through specific interactions between some subunits of the protein. The dimeric form of ATP synthase provides the protein with a spontaneous curvature that sustain their arrangement at the rim of the high-curvature edges of mitochondrial membrane (cristae). Also, a direct interaction with cardiolipin, a lipid present in the inner mitochondrial membrane, induces the dimerization of ATP synthase molecules along cristae. The deletion of those biochemical interactions abolishes the protein dimerization producing an altered mitochondrial function and morphology. Mechanically, membrane bending is one of the key deformation modes by which mitochondrial membranes can be shaped. In particular, bending rigidity and spontaneous curvature are important physical factors for membrane remodelling. Here, we discuss a complementary mechanism whereby the rotatory movement of the ATP synthase might modify the mechanical properties of lipid bilayers and contribute to the formation and regulation of the membrane invaginations