54 research outputs found
Solvent-mediated interactions between nanoparticles at fluid interfaces
We investigate the solvent mediated interactions between nanoparticles
adsorbed at a liquid-vapor interface in comparison to the solvent mediated
interactions in the bulk liquid and vapor phases of a Lennard-Jones solvent.
Molecular dynamics simulation data for the latter are in good agreement with
results from integral equations in the reference functional approximation and a
simple geometric approximation. Simulation results for the solvent mediated
interactions at the interface differ markedly from the interactions of the
particles in the corresponding bulk phases. We find that at short interparticle
distances the interactions are considerably more repulsive than those in either
bulk phase. At long interparticle distances we find evidence for a long-ranged
attraction. We discuss these observations in terms of interfacial interactions,
namely, the three-phase line tension that would operate at short distances, and
capillary wave interactions for longer interparticle distances.Comment: 22 pages, 6 figure
The impact of the interfacial Kapitza resistance on colloidal thermophoresis
Thermal gradients impart a force on colloidal particles pushing the colloids
towards cold or hot regions, a phenomenon called thermophoresis. Existing
theories describe thermophoresis by considering the local perturbation of the
thermal field around the colloid. While these approaches incorporate
interfacial surface free energies, they have consistently ignored the impact of
the Kapitza resistance associated with the colloid-solvent interface. We
propose a theoretical approach to include interfacial Kapitza resistance
effects, and we test the new equations using non-equilibrium molecular dynamics
simulations. We demonstrate that the Kapitza resistance influences the local
thermal field around a colloid, modifying the Soret coefficient. We conclude
that interfacial thermal conductance effects must be included to describe
thermophoresis.Comment: Main paper/: 6 pages, 4 figures; Supplementary: 6 pages, 6 figure
On the Thermodynamic Efficiency of Ca2+-ATPase Molecular Machines
AbstractExperimental studies have shown that the activity of the reconstituted molecular pump Ca2+-ATPase strongly depends on the thickness of the supporting bilayer. It is thus expected that the bilayer structure will have an impact on the thermodynamic efficiency of this nanomachine. Here, we introduce a nonequilibrium-thermodynamics theoretical approach to estimate the thermodynamic efficiency of the Ca2+-ATPase from analysis of available experimental data about ATP hydrolysis and Ca2+ transport. We find that the entropy production, i.e., the heat released to the surroundings under working conditions, is approximately constant for bilayers containing phospholipids with hydrocarbon chains of 18â22 carbon atoms. Our estimates for the heat released during the pump operation agree with results obtained from separate calorimetric experiments on the Ca2+-ATPase derived from sarcoplasmic reticulum. We show that the thermodynamic efficiency of the reconstituted Ca2+-ATPase reaches a maximum for bilayer thicknesses corresponding to maximum activity. Surprisingly, the estimated thermodynamic efficiency is very low, âź12%. We discuss the significance of this result as representative of the efficiency of other nanomachines, and we address the influence of the experimental set-up on such a low efficiency. Overall, our approach provides a general route to estimate thermodynamic efficiencies and heat dissipation in experimental studies of nanomachines
Thermophoretic torque in colloidal particles with mass asymmetry
We investigate the response of anisotropic colloids suspended in a fluid under a thermal field. Using nonequilibrium molecular dynamics computer simulations and nonequilibrium thermodynamics theory, we show that an anisotropic mass distribution inside the colloid rectifies the rotational Brownian motion and the colloids experience transient torques that orient the colloid along the direction of the thermal field. This physical effect gives rise to distinctive changes in the dependence of the Soret coefficient with colloid mass, which features a maximum, unlike the monotonic increase of the thermophoretic force with mass observed in homogeneous colloids
Newton black films as wetting systems
Newton black films (NBFs) can appear under a wide range of experimental conditions. NBFs define the adhesive states of foams and emulsions, showing their formation is a very general physical phenomenon. We show that the existence of NBFs and their whole experimental behavior can be understood within the theory of wetting transitions. NBFs are experimental realizations of partial wetting or pre-wetting states. Hence, they provide experimental systems to investigate the pre-wetting transition, and the spreading behavior under conditions that are very difficult to realize in other experimental systems. We also introduce two new computational approaches to obtain the disjoining pressure isotherm from canonical simulations, and to estimate the contact angles of droplets of nanoscopic dimensionsFinancial support for this work was provided by The Royal Society project âIntrinsic Structure of Aqueous Interfacesâ and the DirecciĂłn General de InvestigaciĂłn, Ministerio
de Ciencia y TecnologĂa of Spain, under Grant No. FIS2010-22047-C05, and by the Comunidad AutĂłnoma de Madrid under the R&D Program of activities MODELICOCM/
S2009ESP-1691. H.M. would like to thank the Universidad
Autonoma de Madrid for the award of a FPU-UAM
doctoral grant. F.B. would like to thank the EPSRC for the
award of a Leadership Fellowship (EP/J003859/1
- âŚ