Optimizing Work Extraction in the Presence of an Entropic Potential: An Entropic Stochastic Resonance.

We study the nontrivial thermodynamic responses of an overdamped Brownian system driven by an unbiased driving force when the particle is confined inside a bilobal irregular structure. The spatial irregularity of the confinement results in an effective entropic bistable potential along the direction of transport. We calculate the thermodynamic response functions in terms of the averaged work done and the absorbed heat over a cycle of driving. We find that the thermodynamic responses are influenced by the nonlinearity of the effective entropic potential, the frequency of the external periodic driving force, and the random thermal fluctuations in a nontrivial way. In the presence of an optimal amount of thermal noise and a favoring driving frequency, the process exhibits a resonance-like precedent in terms of both output work and absorbed heat. We explore the conditions to get best synchronized work extraction (or absorbed heat), which can be utilized as a potential quantifier of an entropic stochastic resonance phenomenon. Finally, we identify a hallmark of entropy dominance over an analogous energy-driven scenario in terms of output work.

3D flower-like zirconium magnesium oxide nanocomposite for efficient fluoride removal.

A 3D flower-shaped bimetallic nanocomposite zirconium magnesium oxide (ZMO) was prepared first time by the controlled solution combustion method using triethanolamine (TEA) as a fuel and chelating agent. The composite material was used to remove excess fluoride via adsorption. The thermal stability of the adsorbent was characterized by thermogravimetric analysis (TGA). Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) were used to characterize the adsorbent. The surface charge of the nano adsorbent was determined by Zeta Sizer. The surface area and pore volume of the adsorbent were determined by Brunauer-Emmett-Teller (BET) isotherm and Barrett-Joyner-Halenda (BJH) methods. The adsorption behavior of fluoride was studied systematically varying the pH, contact time, adsorbent dose, and initial fluoride concentration. The adsorption followed the Langmuir isotherm model with a maximum adsorption capacity of 42.14 mg/g. The pseudo-second-order kinetic model was confirmed by the adsorption study. The maximum adsorption efficiency was in the 6-10 pH range. The reaction mechanism was mainly based on ion exchange between hydroxy and fluoride ions which was proven by X-ray photoelectron spectroscopy (XPS). Real water tests indicated that ZMO could be used as a potential defluoridation agent for fluoride containing groundwater treatment.

Modulation-doping a correlated electron insulator.

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2-especially without inducing structural changes-has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 1021 cm-3 without inducing any measurable structural changes. We find that the MIT temperature (TMIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

Rate enhancement of gated drift-diffusion process by optimal resetting.

"Gating" is a widely observed phenomenon in biochemistry that describes the transition between the activated (or open) and deactivated (or closed) states of an ion-channel, which makes transport through that channel highly selective. In general, gating is a mechanism that imposes an additional restriction on a transport, as the process ends only when the "gate" is open and continues otherwise. When diffusion occurs in the presence of a constant bias to a gated target, i.e., to a target that switches between an open and a closed state, the dynamics essentially slow down compared to ungated drift-diffusion, resulting in an increase in the mean completion time, ⟨TG⟩ > ⟨T⟩, where T denotes the random time of transport and G indicates gating. In this work, we utilize stochastic resetting as an external protocol to counterbalance the delay due to gating. We consider a particle in the positive semi-infinite space that undergoes drift-diffusion in the presence of a stochastically gated target at the origin and is moreover subjected to rate-limiting resetting dynamics. Calculating the minimal mean completion time ⟨Tr⋆G⟩ rendered by an optimal resetting rate r⋆ for this exactly solvable system, we construct a phase diagram that owns three distinct phases: (i) where resetting can make gated drift-diffusion faster even compared to the original ungated process, ⟨Tr⋆G⟩<⟨T⟩<⟨TG⟩, (ii) where resetting still expedites gated drift-diffusion but not beyond the original ungated process, ⟨T⟩≤⟨Tr⋆G⟩<⟨TG⟩, and (iii) where resetting fails to expedite gated drift-diffusion, ⟨T⟩<⟨TG⟩≤⟨Tr⋆G⟩. We also highlight various non-trivial behaviors of the completion time as the resetting rate, gating parameters, and geometry of the set-up are carefully ramified. Gated drift-diffusion aptly models various stochastic processes such as chemical reactions that exclusively take place in certain activated states of the reactants. Our work predicts the conditions under which stochastic resetting can act as a useful strategy to enhance the rate of such processes without compromising their selectivity.

Harnessing the hydrogen evolution reaction (HER) through the electrical mobility of an embossed Ag(I)-molecular cage and a Cu(II)-coordination polymer.

A structurally characterized porous Ag(I)-molecular cage AgMOC and a Cu(II)-coordination polymer CuCP with a pre-synthesized ligand 1,3-bis(((E)-2-methoxybenzylidene)amino)propan-2-ol and its parental amine with thiocyanate are reported to harness electrical mobility-driven hydrogen evolution activity. Porosity-induced electrically conductive AgMOC emerges as a better electrocatalyst with a Tafel slope of 104 mV per decade over Cu(II)-polymer's slope of 128 mV per decade. The electrochemical stability and durability of the designed electrocatalysts in harnessing the HER activity are also examined under experimental conditions.

Geometric Brownian information engine: Essentials for the best performance.

We investigate a geometric Brownian information engine (GBIE) in the presence of an error-free feedback controller that transforms the information gathered on the state of Brownian particles entrapped in monolobal geometric confinement into extractable work. Outcomes of the information engine depend on the reference measurement distance x_{m}, the feedback site x_{f}, and the transverse force G. We determine the benchmarks for utilizing the available information in an output work and the optimum operating requisites for best achievable work. Transverse bias force (G) tunes the entropic contribution in the effective potential and hence the standard deviation (σ) of the equilibrium marginal probability distribution. We recognize that the amount of extractable work reaches a global maximum when x_{f}=2x_{m} with x_{m}∼0.6σ, irrespective of the extent of the entropic limitation. Because of the higher loss of information during the relaxation process, the best achievable work of a GBIE is lower in an entropic system. The feedback regulation also bears the unidirectional passage of particles. The average displacement increases with growing entropic control and is maximum when x_{m}∼0.81σ. Finally, we explore the efficacy of the information engine, a quantity that regulates the efficiency in utilizing the information acquired. With x_{f}=2x_{m}, the maximum efficacy reduces with increasing entropic control and shows a crossover from 2 to 11/9. We discover that the condition for the best efficacy depends only on the confinement lengthscale along the feedback direction. The broader marginal probability distribution accredits the increased average displacement in a cycle and the lower efficacy in an entropy-dominated system.

One-pot solution combustion synthesis of porous spherical-shaped magnesium zinc binary oxide for efficient fluoride removal and photocatalytic degradation of methylene blue and Congo red dye.

A novel porous spherical-shaped magnesium zinc binary oxide (MZO) was successfully prepared for the first time using a chemical process for fluoride removal and photocatalytic methylene blue (MB) and Congo red (CR) dye degradation. XRD, FESEM, and TEM were studied for phase formation, topographic, crystallographic, and detailed structural information. The surface charge and optical properties of the adsorbent were studied by zeta potential and photoluminescence spectra. The synthesized nano-adsorbents showed high fluoride removal capacity (43.10 mg/g) and photocatalytic activity with a degradation efficiency of 97.83% and 78.40% for MB and CR, respectively. The adsorption was strongly pH-dependent and worked well in the range 6-9. The kinetic studies were performed for both fluoride removal and dye degradation and were found to follow pseudo-second-order and first-order rate law, respectively. The samples were found to be extremely reusable and selective for fluoride removal in presence of co-ions such as NO3-, SO42-, and Cl-. The basic fluoride adsorption process of the samples can be related to ion exchange and electrostatic interactions, according to XPS and FTIR data. The detailed mechanistic study of photocatalytic dye degradation showed that the reaction occurred via OH radicals. Thus, MZO could be considered an effective and quick adsorbent for water purification in fluoride-containing groundwater and industrial dye wastewater.

Geometric Brownian information engine: Upper bound of the achievable work under feedback control.

We design a geometric Brownian information engine by considering overdamped Brownian particles inside a two-dimensional monolobal confinement with irregular width along the transport direction. Under such detention, particles experience an effective entropic potential which has a logarithmic form. We employ a feedback control protocol as an outcome of error-free position measurement. The protocol comprises three stages: measurement, feedback, and relaxation. We reposition the center of the confinement to the measurement distance (xp) instantaneously when the position of the trapped particle crosses xp for the first time. Then, the particle is allowed for thermal relaxation. We calculate the extractable work, total information, and unavailable information associated with the feedback control using this equilibrium probability distribution function. We find the exact analytical value of the upper bound of extractable work as (53-2ln2)kBT. We introduce a constant force G downward to the transverse coordinate (y). A change in G alters the effective potential of the system and tunes the relative dominance of entropic and energetic contributions in it. The upper bound of the achievable work shows a crossover from (53-2ln2)kBT to 12kBT when the system changes from an entropy-dominated regime to an energy-dominated one. Compared to an energetic analog, the loss of information during the relaxation process is higher in the entropy-dominated region, which accredits the less value in achievable work. Theoretical predictions are in good agreement with the Langevin dynamics simulation studies.

Escape kinetics of self-propelled particles from a circular cavity.

We numerically investigate the mean exit time of an inertial active Brownian particle from a circular cavity with single or multiple exit windows. Our simulation results witness distinct escape mechanisms depending on the relative amplitudes of the thermal length and self-propulsion length compared to the cavity and pore sizes. For exceedingly large self-propulsion lengths, overdamped active particles diffuse on the cavity surface, and rotational dynamics solely governs the exit process. On the other hand, the escape kinetics of a very weakly damped active particle is largely dictated by bouncing effects on the cavity walls irrespective of the amplitude of self-propulsion persistence lengths. We show that the exit rate can be maximized for an optimal self-propulsion persistence length, which depends on the damping strength, self-propulsion velocity, and cavity size. However, the optimal persistence length is insensitive to the opening windows' size, number, and arrangement. Numerical results have been interpreted analytically based on qualitative arguments. The present analysis aims at understanding the transport controlling mechanism of active matter in confined structures.

Noise-induced symmetry breaking of self-regulators: Nonequilibrium transition toward homochirality.

We present a theoretical model to study the origin of chiral symmetry breaking of a racemic mixture of optically active biomolecules. We consider a collection of Brownian particles, which can stay in any of the three possible isomeric states: one achiral and two enantiomers. Isomers are undergoing self-regulatory reaction along with chiral inhibition and achiral decay processes. The reaction rates of the isomeric states are guided by their neighbors as well as the thermal fluctuations of the system. We find that an alteration in the relative dominance of self-regulation, chiral inhibition, and achiral decay processes breaks the chiral symmetry of the system, which is either partial or complete. This results in four different asymmetric population states, viz., three-isomer coexistence, enantiomeric coexistence, chiral-achiral coexistence, and homochiral state. A change in the reaction condition induces nonequilibrium transition among these states. We also report that a fast stochastic self-regulation and a slow chiral inhibition and achiral decay process along with a threshold population of interacting neighbors suffice for the requisite for transition toward a completely symmetry broken state, i.e., homochirality.

Pseudochemotaxis in inhomogeneous active Brownian systems.

We study dynamical properties of confined, self-propelled Brownian particles in an inhomogeneous activity profile. Using Brownian dynamics simulations, we calculate the probability to reach a fixed target and the mean first passage time to the target of an active particle. We show that both these quantities are strongly influenced by the inhomogeneous activity. When the activity is distributed such that high-activity zone is located between the target and the starting location, the target finding probability is increased and the passage time is decreased in comparison to a uniformly active system. Moreover, for a continuously distributed profile, the activity gradient results in a drift of active particle up the gradient bearing resemblance to chemotaxis. Integrating out the orientational degrees of freedom, we derive an approximate Fokker-Planck equation and show that the theoretical predictions are in very good agreement with the Brownian dynamics simulations.

Experimental Determination of η/s for Finite Nuclear Matter.

We present, for the first time, simultaneous determination of shear viscosity (η) and entropy density (s) and thus, η/s for equilibrated nuclear systems from A∼30 to A∼208 at different temperatures. At finite temperature, η is estimated by utilizing the γ decay of the isovector giant dipole resonance populated via fusion evaporation reaction, while s is evaluated from the nuclear level density parameter (a) and nuclear temperature (T), determined precisely by the simultaneous measurements of the evaporated neutron energy spectra and the compound nuclear angular momenta. The transport parameter η and the thermodynamic parameter s both increase with temperature, resulting in a mild decrease of η/s with temperature. The extracted η/s is also found to be independent of the neutron-proton asymmetry at a given temperature. Interestingly, the measured η/s values are comparable to that of the high-temperature quark-gluon plasma, pointing towards the fact that strong fluidity may be the universal feature of the strong interaction of many-body quantum systems.

Functionalization of the Polymeric Surface with Bioceramic Nanoparticles via a Novel, Nonthermal Dip Coating Method.

The only nonthermal method of depositing a bioceramic-based coating on polymeric substrates is by incubation in liquid, e.g., simulated body fluid to form an apatite-like layer. The drawbacks of this method include the long processing time, the production of low scratch resistant coating, and an end product that does not resemble the intended bioceramic composition. Techniques, such as plasma spraying and magnetron sputtering, involving high processing temperature are unsuitable for polymers, e.g., PMMA. Here, we introduce a nonthermal coating method to immobilize hydroxyapatite (HAp) and TiO2 nanoparticles on PMMA via a simple and fast dip coating method. Cavities that formed on the PMMA, induced by chloroform, appeared to trap the nanoparticles which accumulated to form layers of bioceramic coating only after 60 s. The resulting coating was hydrophilic and highly resistant to delamination. In the context of our research and to address the current clinical need, we demonstrate that the HAp-coated PMMA, which is intended to be used as a visual optic of a corneal prosthetic device, improves its bonding and biointegration with collagen, the main component of a corneal stroma. The HAp-coated PMMA resulted in better adhesion with the collagen than untreated PMMA in artificial tear fluid over 28 days. Human corneal stromal fibroblasts showed better attachment, viability, and proliferation rate on the HAp-coated PMMA than on untreated PMMA. This coating method is an innovative solution to immobilize various bioceramic nanoparticles on polymers and may be used in other biomedical implants.

Ratchet rectification effect on the translocation of a flexible polyelectrolyte chain.

We report a three dimensional Langevin dynamics simulation of a uniformly charged flexible polyelectrolyte chain, translocating through an asymmetric narrow channel with periodically varying cross sections under the influence of a periodic external electric field. When reflection symmetry of the channel is broken, a rectification effect is observed with a favored direction for the chain translocation. For a given volume of the channel unit and polymer length, the rectification occurs below a threshold frequency of the external periodic driving force. We have also observed that the extent of the rectification varies non-monotonically with increasing molecular weight and the strength of geometric asymmetry of the channel. Observed non-monotonicity of the rectification performance has been interpreted in terms of a competition between two effects arising from the channel asymmetry and change in conformational entropy. An analytical model is presented with predictions consistent with the simulation results.

Stochastic resonance during a polymer translocation process.

We have studied the occurrence of stochastic resonance when a flexible polymer chain undergoes a single-file translocation through a nano-pore separating two spherical cavities, under a time-periodic external driving force. The translocation of the chain is controlled by a free energy barrier determined by chain length, pore length, pore-polymer interaction, and confinement inside the donor and receiver cavities. The external driving force is characterized by a frequency and amplitude. By combining the Fokker-Planck formalism for polymer translocation and a two-state model for stochastic resonance, we have derived analytical formulas for criteria for emergence of stochastic resonance during polymer translocation. We show that no stochastic resonance is possible if the free energy barrier for polymer translocation is purely entropic in nature. The polymer chain exhibits stochastic resonance only in the presence of an energy threshold in terms of polymer-pore interactions. Once stochastic resonance is feasible, the chain entropy controls the optimal synchronization conditions significantly.

Surface Modification of PMMA to Improve Adhesion to Corneal Substitutes in a Synthetic Core-Skirt Keratoprosthesis.

Patients with advanced corneal disease do poorly with conventional corneal transplantation and require a keratoprosthesis (KPro) for visual rehabilitation. The most widely used KPro is constructed using poly(methyl methacrylate) (PMMA) in the central optical core and a donor cornea as skirt material. In many cases, poor adherence between the PMMA and the soft corneal tissue is responsible for device "extrusion" and bacterial infiltration. The interfacial adhesion between the tissue and the PMMA was therefore critical to successful implantation and device longevity. In our approach, we modified the PMMA surface using oxygen plasma (plasma group); plasma followed by calcium phosphate (CaP) coating (p-CaP); dopamine followed by CaP coating (d-CaP); or plasma followed by coating with (3-aminopropyl)triethoxysilane (3-APTES). To create a synthetic KPro model, we constructed and attached 500 µm thick collagen type I hydrogel on the modified PMMA surfaces. Surface modifications produced significantly improved interfacial adhesion strength compared to untreated PMMA (p < 0.001). The p-CaP group yielded the best interfacial adhesion with the hydrogel (177 ± 27 mN/cm(2)) followed by d-CaP (168 ± 31 mN/cm(2)), 3-APTES (145 ± 12 mN/cm(2)), and plasma (119 ± 10 mN/cm(2)). Longer-term stability of the adhesion was achieved by d-CaP, which, after 14 and 28 days of incubation in phosphate buffered saline, yielded 164 ± 25 mN/cm(2) (p = 0.906 compared to adhesion at day 1) and 131 ± 20 mN/cm(2) (p = 0.053), respectively. In contrast, significant reduction of adhesion strength was observed in p-CaP group over time (p < 0.001). All surface coatings were biocompatible to human corneal stromal fibroblasts, except for the 3-APTES group, which showed no live cells at 72 h of culture. In contrast, cells on d-CaP surface showed good anchorage, evidenced by the expression of focal adhesion complex (paxillin and vinculin), and prominent filopodia protrusions. In conclusion, d-CaP can not only enhance and provide stability to the adhesion of collagen hydrogel on the PMMA surface but also promote biointegration.

Collagen-Based Artificial Corneal Scaffold with Anti-Infective Capability for Prevention of Perioperative Bacterial Infections.

Bacterial infection following implantation of an artificial corneal scaffold is a serious complication. Conventional antibiotic prophylaxis, which includes topical vancomycin application, is limited by low bioavailability, high dosing requirement, and poor patient compliance. The ideal option to overcome these issues is an antibiotic-eluting corneal prosthesis that sustains the local release of drug. In this study, we incorporated vancomycin in thick 15% collagen hydrogels to create an artificial corneal scaffold with anti-infective capability. The incorporation of vancomycin did not significantly alter the Young's modulus, transparency and refractive index of the vancomycin-loaded hydrogel (VH), which were 0.79 ± 0.04 MPa (p = 0.233 compared to blank hydrogel), 94.3 ± 2.3% (p = 0.115) and 1.346 ± 0.005 (p = 0.264), respectively. In vitro, the drug elution was sustained for up to 7 days. The VH was subsequently implanted intrastromally in rabbit corneas, replacing stromal tissue that was removed following femtosecond laser-assisted small incision lenticule extraction procedure. In vivo, the vancomcyin could be detected in the aqueous humor for up to 10 days. We then created a corneal infectious keratitis model by intrastromal injection of 1 × 108 CFU/ml of Staphylococcus aureus inoculate on day 2 postimplantation. On day 3 postinfection, the VH-implanted corneas were clear and nonedematous and showed a substantial reduction of log 2.5 in S. aureus compared to the blank hydrogel-implanted corneas, which appeared hazy, edematous, and had excessive inflammation. Immunohistochemistry of inflammatory marker, CD18, demonstrated a significant reduction in inflammatory cells in VH-implanted corneas (49 ± 9 cells/unit area) compared to blank hydrogel-implanted corneas (523 ± 15 cells/unit area) (p < 0.001). In conclusion, we have demonstrated the efficacy of a drug-eluting corneal implant in preventing perioperative bacterial infections.

Systematic reverse engineering of network topologies: a case study of resettable bistable cellular responses.

A focused theme in systems biology is to uncover design principles of biological networks, that is, how specific network structures yield specific systems properties. For this purpose, we have previously developed a reverse engineering procedure to identify network topologies with high likelihood in generating desired systems properties. Our method searches the continuous parameter space of an assembly of network topologies, without enumerating individual network topologies separately as traditionally done in other reverse engineering procedures. Here we tested this CPSS (continuous parameter space search) method on a previously studied problem: the resettable bistability of an Rb-E2F gene network in regulating the quiescence-to-proliferation transition of mammalian cells. From a simplified Rb-E2F gene network, we identified network topologies responsible for generating resettable bistability. The CPSS-identified topologies are consistent with those reported in the previous study based on individual topology search (ITS), demonstrating the effectiveness of the CPSS approach. Since the CPSS and ITS searches are based on different mathematical formulations and different algorithms, the consistency of the results also helps cross-validate both approaches. A unique advantage of the CPSS approach lies in its applicability to biological networks with large numbers of nodes. To aid the application of the CPSS approach to the study of other biological systems, we have developed a computer package that is available in Information S1.

Resonant activation in a colored multiplicative thermal noise driven closed system.

In this paper, we have demonstrated that resonant activation (RA) is possible even in a thermodynamically closed system where the particle experiences a random force and a spatio-temporal frictional coefficient from the thermal bath. For this stochastic process, we have observed a hallmark of RA phenomena in terms of a turnover behavior of the barrier-crossing rate as a function of noise correlation time at a fixed noise variance. Variance can be fixed either by changing temperature or damping strength as a function of noise correlation time. Our another observation is that the barrier crossing rate passes through a maximum with increase in coupling strength of the multiplicative noise. If the damping strength is appreciably large, then the maximum may disappear. Finally, we compare simulation results with the analytical calculation. It shows that there is a good agreement between analytical and numerical results.

Polycaprolactone-thiophene-conjugated carbon nanotube meshes as scaffolds for cardiac progenitor cells.

The myocardium is unable to regenerate itself after infarct, resulting in scarring and thinning of the heart wall. Our objective was to develop a patch to buttress and bypass the scarred area, while allowing regeneration by incorporated cardiac stem/progenitor cells (CPCs). Polycaprolactone (PCL) was fabricated as both sheets by solvent casting, and fibrous meshes by electrospinning, as potential patches, to determine the role of topology in proliferation and phenotypic changes to the CPCs. Thiophene-conjugated carbon nanotubes (T-CNTs) were incorporated to enhance the mechanical strength. We showed that freshly isolated CPCs from murine hearts neither attached nor spread on the PCL sheets, both with and without T-CNT. As electrospun meshes, however, both PCL and PCL/T-CNT supported CPC adhesion, proliferation, and differentiation. The incorporation of T-CNT into PCL resulted in a significant increase in mechanical strength but no morphological changes to the meshes. In turn, proliferation, but not differentiation, of CPCs into cardiomyocytes was enhanced in T-CNT containing meshes. We have shown that changing the topology of PCL, a known hydrophobic material, dramatically altered its properties, in this case, allowing CPCs to survive and differentiate. With further development, PCL/T-CNT meshes or similar patches may become a viable strategy to aid restoration of the postmyocardial infarction myocardium.