Marian Manciu

Specific ion effects via ion hydration: I.Surface tension

A simple modality is suggested to include, in the framework of a modified Poisson-Boltzmann approach, specific ion effects via the change in the ion hydration between the bulk and the vicinity of the surface. This approach can account for both the depletion of the interfacial region of structure-making ions as well as for the accumulation of structure-breaking ions near the interface. Expressions for the change in interfacial tension as a function of electrolyte concentrations are derived. On the basis of this theory, one explains the dependence of the surface potential on pH and electrolyte concentration, the existence of a minimum in the surface tension at low electrolyte concentrations and the linear dependence, with a positive or sometimes negative slope, of the surface tension on the electrolyte concentration at sufficiently high ionic strengths.

Impulsive propagation in dissipative and disordered chains with power-law repulsive potentials

Abstract We report particle dynamics based studies of impulse propagation in a chain of elastic beads with dissipative contacts and with randomly distributed masses. The interaction between the beads is characterized by the potential V ( δ )∼ δ n , δ ≥0 being grain overlap, n >2 and at zero external loading, i.e., under conditions of “sonic vacuum” in which sound cannot propagate through the chain [J. Appl. Mech. Technol. Phys. 5 (1983) 733]. In the earlier work, we have confirmed the studies of Nesterenko and coworkers and have reported that impulses propagate as solitary waves in the system of interest in the absence of dissipation and disorder [Physica A 268 (1999) 644]. In the present study, we first discuss the effects of restitution and velocity dependent friction on the propagation of the impulse. We next report that the maximum energy E max of the solitary wave as it propagates from a chain of monodisperse grains of mass m to a chain with masses m (1+ r ( z ) ϵ ), where −1≤ r ( z )≤1 and ϵ =const. that measures the degree of randomness, decays with linear distance traveled z as exp(− α E z ), α E ∼ ϵ 2+ f ( n ) , f ( n ) being some n dependent constant for 2 n monodisperse chains, the velocity of the solitary wave c ∼ E max ( n −2)/2 n . In polydisperse chains, we show that the propagation speed of a non-dispersive solitary wave decays with distance as exp(− α c z ), where α c = α E ( n −2)/2 n .

DISCRETE HERTZIAN CHAINS AND SOLITONS

We consider elastic grains which repel upon contact according to the Hertz potential V=aδn,n>2, where δ is the overlap between adjacent grains and report numerical and analytical studies for the case of a 1-D chain of spherical grains to show that an impulse initiated at an end of a chain of Hertzian grains in contact, eventually propagates as a soliton for all n>2.

Thermalizing an impulse

We study the propagation of an impulse through a finite chain of N elastic beads in which the grain diameters progressively shrink by a factor q. We show that it should be possible to construct “tapered” chains that can effectively thermalize shock waves.

The propagation and backscattering of soliton-like pulses in a chain of quartz beads and related problems. (II). Backscattering

We demonstrate that the propagation of solitons, soliton-like excitations and acoustic pulses discussed in the preceding article (M. Manciu, S. Sen and A.J. Hurd, Physica A, preceding article) can be used to detect buried impurities in a chain of elastic grains with Hertzian contacts. We also present preliminary data for 3D granular beds, where soliton-like objects can form and can be used to probe for buried impurities, thus suggesting that soliton-pulse spectroscopy has the potential to become a valuable tool for probing the structural properties of granular assemblies. The effects of restitution are briefly discussed. We refer to available experiments which support our contention.

The propagation and backscattering of soliton-like pulses in a chain of quartz beads and related problems. (I). Propagation

We confirm that for vanishingly small loading and large impact condition, it may be possible to generate solitons in a chain of grains that are characterized by Hertzian contacts. For uniform or progressive loading conditions throughout the chain, one generates soft-solitons which are weakly dispersive in space and time. Under conditions of weak impact, one generates acoustic pulses through the chain. We describe the displacements, velocities and accelerations suffered by the individual grains when subjected to solitons, soft-solitons and acoustic pulses and describe the effects of restitution on the propagating pulse.

Dynamics of a gravitationally loaded chain of elastic beads

Elastic beads repel in a highly nonlinear fashion, as described by Hertz law, when they are compressed against one another. Vertical stacking results in significant compressions of beads at finite distances from the surface of the stack due to gravity. Analytic studies that have been reported in the literature assume acoustic excitations upon weak perturbation [J. Hong et al., Phys. Rev. Lett. 82, 3058 (1999)] and soliton-like excitations upon strong perturbation [V. Nesterenko, J. Appl. Mech. Tech. Phys. 5, 733 (1983); S. Sen and M. Manciu, Physica A 268, 644 (1999)]. The present study probes the position, velocity and acceleration and selected two-point temporal correlations and their power spectra for individual beads for cases in which the system has been (i) weakly, (ii) strongly, and (iii) moderately perturbed at the surface in the sense specified in the text. Our studies reveal the existence of distinctly different dynamical behavior of the tagged beads, in contrast to conventional acoustic response, as the strength of the perturbation is varied at fixed gravitational loading. We also comment on the effects of polydispersity on system dynamics and probe the relaxation of isolated light and heavy beads in the chain. (c) 2000 American Institute of Physics.

Ejection of ferrofluid grains using nonlinear acoustic impulses— A particle dynamical study

We consider a model dilute ferrofluid with the grains suspended in water (e.g.,γ-Fe2O3) and subject the system to a strong, homogeneous magnetic field directed perpendicular to the surface such that there is chain formation along the field direction. We show that an appropriate impulse initiated at the base of the container might travel as a nondispersive soliton pulse with sufficient energy to overcome surface tension and eject the ferrofluid grain nearest to the liquid–air interface. The proposed mechanism, if successfully realized in the laboratory, could help design a nozzle-free, ink-jet printer of unparalleled resolution.We consider a model dilute ferrofluid with the grains suspended in water (e.g.,γ-Fe2O3) and subject the system to a strong, homogeneous magnetic field directed perpendicular to the surface such that there is chain formation along the field direction. We show that an appropriate impulse initiated at the base of the container might travel as a nondispersive soliton pulse with sufficient energy to overcome surface tension and eject the ferrofluid grain nearest to the liquid–air interface. The proposed mechanism, if successfully realized in the laboratory, could help design a nozzle-free, ink-jet printer of unparalleled resolution.

Impact of CTLA-4 blockade in conjunction with metronomic chemotherapy on preclinical breast cancer growth

Background:Although there are reports that metronomic cyclophosphamide (CTX) can be immune stimulating, the impact of its combination with anti-CTLA-4 immunotherapy for the treatment of cancer remains to be evaluated.Methods:Murine EMT-6/P breast cancer, or its cisplatin or CTX-resistant variants, or CT-26 colon, were implanted into Balb/c mice. Established tumours were monitored for relative growth following treatment with anti-CTLA-4 antibody alone or in combination with; (a) metronomic CTX (ldCTX; 20 mg kg−1 day−1), b) bolus (150 mg kg−1) plus ldCTX, or (c) sequential treatment with gemcitabine (160 mg kg−1 every 3 days).Results:EMT-6/P tumours responded to anti-CTLA-4 therapy, but this response was less effective when combined with bolus plus ldCTX. Anti-CTLA-4 could be effectively combined with either ldCTX (without a bolus), or with regimens of either sequential or concomitant gemcitabine, including in orthotopic EMT-6 tumours, and independently of the schedule of drug administration. Tumour responses were confirmed with CT-26 tumours but were less pronounced in drug-resistant EMT-6/CTX or EMT-6/DDP tumour models than in the parent tumour. A number of tumour bearing mice developed spontaneous metastases under continuous therapy. The majority of cured mice rejected tumour re-challenges.Conclusions:Metronomic CTX can be combined with anti-CTLA-4 therapy, but this therapy is impaired by concomitant bolus CTX. Sequential therapy of anti-CTLA-4 followed by gemcitabine is effective in chemotherapy-naive tumours, although tumour relapses can occur, in some cases accompanied by the development of spontaneous metastases.

On the surface tension and Zeta potential of electrolyte solutions

The distribution of ions in the vicinity of the air/water interface is still a matter of strong debate, with numerous calculations and experiments providing contradictory results, even regarding the preference of simple ions (such as H+ and OH-) for interfacial or bulk water. When short range interactions between ions and the interface are assumed independent of bulk concentrations, if they are compatible with the surface tension data, they underpredict the experimental Zeta potentials by orders of magnitude. If they are compatible with Zeta potential data, they are in strong disagreement with surface tension experiments. It is suggested that these observations might be a result of the relatively low number of interfacial water molecules available to hydrate the ions and the competition between various ions for adsorption sites. Therefore, whereas at low bulk concentrations, the Structure-Breaking ions prefer the interface, at sufficiently large bulk concentrations the surface adsorptions of these ions become saturated, and their interfacial concentrations may become lower than in the bulk. Consequently, the total interactions of ions with the interface can be strongly attractive at low bulk concentrations, and less attractive (or even repulsive), at high concentrations. To model this effect, the interactions between ions and interface are taken into account via modified Langmuir adsorption expressions for OH- and Cl-, while the H+ ions are considered to be attached to any interfacial water molecule, even if the latter participate in the hydration of anions. The simple model of adsorption employed here is in agreement with both experiments on Zeta potential and on surface tension, and might reveal the conditions under which a given ion exhibits propensity for either the air/water interface, or for bulk water.