Antiresonant driven systems for particle manipulation.

We report on the onset of antiresonant behavior of mass transport systems driven by time-dependent forces. Antiresonances arise from the coupling of a sufficiently high number of space-time modes of the force. The presence of forces having a wide space-time spectrum, a necessary condition for the formation of an antiresonance, is typical of confined systems with uneven and deformable walls that induce entropic forces dependent on space and time. We have analyzed, in particular, the case of polymer chains confined in a flexible channel and shown how they can be sorted and trapped. The presence of resonance-antiresonance pairs found can be exploited to design protocols able to engineer optimal transport processes and to manipulate the dynamics of nano-objects.

Extrasynaptic Communication.

Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.

Thermodynamic Efficiency of Somatic Exocytosis of Serotonin.

Through somatic exocytosis neurons liberate immense amounts of transmitter molecules that modulate the functioning of the nervous system. A stream of action potentials triggers an ATP-dependent transport of transmitter-containing vesicles to the plasma membrane, that ends with a large-scale exocytosis. It is commonly assumed that biological processes use metabolic energy with a high thermodynamic efficiency, meaning that most energy generates work with minor dissipation. However, the intricate ultrastructure underlying the pathway for the vesicle flow necessary for somatic exocytosis challenges this possibility. To study this problem here we first applied thermodynamic theory to quantify the efficiency of somatic exocytosis of the vital transmitter serotonin. Then we correlated the efficiency to the ultrastructure of the transport pathway of the vesicles. Exocytosis was evoked in cultured Retzius neurons of the leech by trains of 10 impulses delivered at 20 Hz. The kinetics of exocytosis was quantified from the gradual fluorescence increase of FM1-43 dye as it became incorporated into vesicles that underwent their exo-endocytosis cycle. By fitting a model of the vesicle transport carried by motor forces to the kinetics of exocytosis, we calculated the thermodynamic efficiency of the ATP expenses per vesicle, as the power of the transport divided by total energy ideally produced by the hydrolysis of ATP during the process. The efficiency was remarkably low (0.1-6.4%) and the values formed a W-shape distribution with the transport distances of the vesicles. Electron micrographs and fluorescent staining of the actin cortex indicated that the slopes of the W chart could be explained by the interaction of vesicles with the actin cortex and the calcium-releasing endoplasmic reticulum. We showed that the application of thermodynamic theory permitted to predict aspects of the intracellular structure. Our results suggest that the distribution of subcellular structures that are essential for somatic exocytosis abates the thermodynamic efficiency of the transport by hampering vesicle mobilization. It is remarkable that the modulation of the nervous system occurs at the expenses of an efficient use of metabolic energy.

Understanding Gelation as a Nonequilibrium Self-Assembly Process.

Gel formation is described by a nonequilibrium self-assembly (SA) mechanism which considers the presence of precursors. Assuming that nonequilibrium structures appear and are maintained by entropy production, we developed a mesoscopic nonequilibrium thermodynamic model that describes the dynamic assembly of the structures. In the model, the evolution of the structures from the initially inactivated building blocks to the final agglomerates is governed by kinetic equations of the Fokker-Planck type. From these equations, we get the probability densities which enable one to know the measurable quantities such as the concentrations of the different components and the dynamic structure factor obtained in light-scattering experiments. Our results obtained are in very good agreement with the experiments. The model proposed can in general be used to analyze the kinetics of formation of nonequilibrium SA structures usually found in biomedicine and advanced materials.

Entropy production and rectification efficiency in colloid transport along a pulsating channel.

We study the current rectification of particles moving in a pulsating channel under the influence of an applied force. We have shown the existence of different rectification scenarios in which entropic and energetic effects compete. The effect can be quantified by means of a rectification coefficient that is analyzed in terms of the force, the frequency and the diffusion coefficient. The energetic cost of the motion of the particles expressed in terms of the entropy production depends on the importance of the entropic contribution to the total force. Rectification is more important at low values of the applied force when entropic effects become dominant. In this regime, the entropy production is not invariant under reversal of the applied force. The phenomenon observed could be used to optimize transport in microfluidic devices or in biological channels.

Nonequilibrium self-assembly induced Liesegang rings in a non-isothermal system.

We propose a model to show the formation of Liesegang rings under non-isothermal conditions. The model formulates reaction-diffusion equations for all components intervening in the process together with an evolution equation for the temperature. The reactive parts in these equations follow from the analysis of the non-equilibrium self-assembly (NESA) process undergone by the meso-particles which make up the patterns. The solution of these equations enables us to know the concentration of each component, the spherical structures diameter, and the system temperature as a function of time and radial position. The values found for the structures diameter and the rings position are in agreement with the experiments. The results for the system temperature with peaks at the rings positions suggest that heat accumulates at these positions as a consequence of the dissipation inherent to the NESA process. Our model enables us to rationalize how from non-homogeneous initial conditions a transient self-organization process involving formation of self-assembled structures may produce macroscopic patterns. It can, in general, be used to analyze pattern formation due to diffusion-reaction-precipitation processes with potential applications in the design of advanced materials.

Theory of Casimir Forces without the Proximity-Force Approximation.

We analyze both the attractive and repulsive Casimir-Lifshitz forces recently reported in experimental investigations. By using a kinetic approach, we obtain the Casimir forces from the power absorbed by the materials. We consider collective material excitations through a set of relaxation times distributed in frequency according to a log-normal function. A generalized expression for these forces for arbitrary values of temperature is obtained. We compare our results with experimental measurements and conclude that the model goes beyond the proximity-force approximation.

Anomalous law of cooling.

We analyze the temperature relaxation phenomena of systems in contact with a thermal reservoir that undergoes a non-Markovian diffusion process. From a generalized Langevin equation, we show that the temperature is governed by a law of cooling of the Newton's law type in which the relaxation time depends on the velocity autocorrelation and is then characterized by the memory function. The analysis of the temperature decay reveals the existence of an anomalous cooling in which the temperature may oscillate. Despite this anomalous behavior, we show that the variation of entropy remains always positive in accordance with the second law of thermodynamics.

Khinchin theorem and anomalous diffusion.

A recent Letter [M. H. Lee, Phys. Rev. Lett. 98, 190601 (2007)] has called attention to the fact that irreversibility is a broader concept than ergodicity, and that therefore the Khinchin theorem [A. I. Khinchin, (Dover, New York, 1949)] may fail in some systems. In this Letter we show that for all ranges of normal and anomalous diffusion described by a generalized Langevin equation the Khinchin theorem holds.