24 research outputs found
Coherent quantum transport in the presence of a finite-range transversely polarized time-dependent field
This work investigates the quantum transport in a narrow constriction acted
upon by a finite-range transversely polarized time-dependent electric field. A
generalized scattering-matrix method is developed that has incorporated a
time-dependent mode-matching scheme. The transverse field induces coherent
inelastic scatterings that include both intersubband and intersideband
transitions. These scatterings give rise to the dc conductance a general
suppressed feature that escalates with the chemical potential. In addition,
particular suppressed features -- the dip structures -- are found in . These
features are recognized as the quasi-bound-state (QBS) features that arise from
electrons making intersubband transitions to the vicinity of a subband bottom.
For the case of larger field intensities, the QBS features that involve more
photons are more evident. These QBS features are closely associated with the
singular density of states at the subband bottoms. An experimental setup is
proposed for the observation of these features.Comment: 8 pages, 4 figure
Generation of Porous Particle Structures using the Void Expansion Method
The newly developed "void expansion method" allows for an efficient
generation of porous packings of spherical particles over a wide range of
volume fractions using the discrete element method. Particles are randomly
placed under addition of much smaller "void-particles". Then, the void-particle
radius is increased repeatedly, thereby rearranging the structural particles
until formation of a dense particle packing.
The structural particles' mean coordination number was used to characterize
the evolving microstructures. At some void radius, a transition from an
initially low to a higher mean coordination number is found, which was used to
characterize the influence of the various simulation parameters. For structural
and void-particle stiffnesses of the same order of magnitude, the transition is
found at constant total volume fraction slightly below the random close packing
limit. For decreasing void-particle stiffness the transition is shifted towards
a smaller void-particle radius and becomes smoother.Comment: 9 pages, 8 figure
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
The effect of a time-modulated gate-voltage on the differential conductance
of a saddle-point constriction is studied. The constriction is modeled by a
symmetric saddle-point potential and the time-modulated gate-voltage is
represented by a potential of the form . For less than half of the transverse subband energy
level spacing, gate-voltage-assisted (suppressed) feature occurs when the
chemical potential is less (greater) than but close to the threshold
energy of a subband. As increases, is found to exhibit,
alternatively, the assisted and the suppressed feature. For larger
, these two features may overlap with one another. Dip structures
are found in the suppressed regime. Mini-steps are found in the assisted regime
only when the gate-voltage covers region far enough away from the center of the
constriction.Comment: 8 pages, 6 figure
Tailoring swelling to control softening mechanisms during cyclic loading of PEG/cellulose hydrogel composites
One of the novel approaches for discogenic lower back pain treatment is to permanently replace the core of the intervertebral disc, so-called Nucleus Pulposus, through minimally invasive surgery. Recently, we have proposed Poly(Ethylene Glycol) Dimethacrylate (PEGDM) hydrogel reinforced with Nano-Fibrillated Cellulose (NFC) fibers as an appropriate replacement material. In addition to the tuneable properties, that mimic those of the native tissue, the surgeon can directly inject it into the degenerated disc and cure it in situ via UV-light irradiation. However, in view of clinical applications, the reliability of the proposed material has to be tested under long-term fatigue loading. To that end, the present study focused on the characterization of the fatigue behavior of the composite hydrogel and investigated the governing physical phenomena behind it. The results show that composite PEGDM-NFC hydrogel withstands the 10 million compression cycles at physiological condition. However, its modulus decreases by almost 10% in the first cycle and then remains constant, while cyclic loading does not affect the neat PEGDM hydrogel. The observed softening behavior has similar characteristics of the Mullins effect. It is shown that the reduction of modulus is due to the gradual change of NFC network, which is highly stretched in the swollen state. Moreover, the swelling degree of the matrix is correlated to the extent of softening during cyclic loading. Consequently, softening can be minimized by lowering the swelling of the composite hydrogel
Coherent quantum transport in narrow constrictions in the presence of a finite-range longitudinally polarized time-dependent field
We have studied the quantum transport in a narrow constriction acted upon by
a finite-range longitudinally polarized time-dependent electric field. The
electric field induces coherent inelastic scatterings which involve both
intra-subband and inter-sideband transitions. Subsequently, the dc conductance
G is found to exhibit suppressed features. These features are recognized as the
quasi-bound-state (QBS) features which are associated with electrons making
transitions to the vicinity of a subband bottom, of which the density of states
is singular. Having valley-like instead of dip-like structures, these QBS
features are different from the G characteristics for constrictions acted upon
by a finite-range time-modulated potential. In addition, the subband bottoms in
the time-dependent electric field region are shifted upward by an energy
proportional to the square of the electric field and inversely proportional to
the square of the frequency. This effective potential barrier is originated
from the square of the vector potential and it leads to the interesting
field-sensitive QBS features. An experimental set-up is proposed for the
observation of these features.Comment: 8 pages, 4 figure
Mechanical Responses and Stress Fluctuations of a Supercooled Liquid in a Sheared Non-Equilibrium State
A steady shear flow can drive supercooled liquids into a non-equilibrium
state. Using molecular dynamics simulations under steady shear flow
superimposed with oscillatory shear strain for a probe, non-equilibrium
mechanical responses are studied for a model supercooled liquid composed of
binary soft spheres. We found that even in the strongly sheared situation, the
supercooled liquid exhibits surprisingly isotropic responses to oscillating
shear strains applied in three different components of the strain tensor. Based
on this isotropic feature, we successfully constructed a simple two-mode
Maxwell model that can capture the key features of the storage and loss moduli,
even for highly non-equilibrium state. Furthermore, we examined the correlation
functions of the shear stress fluctuations, which also exhibit isotropic
relaxation behaviors in the sheared non-equilibrium situation. In contrast to
the isotropic features, the supercooled liquid additionally demonstrates
anisotropies in both its responses and its correlations to the shear stress
fluctuations. Using the constitutive equation (a two-mode Maxwell model), we
demonstrated that the anisotropic responses are caused by the coupling between
the oscillating strain and the driving shear flow. We measured the magnitude of
this violation in terms of the effective temperature. It was demonstrated that
the effective temperature is notably different between different components,
which indicates that a simple scalar mapping, such as the concept of an
effective temperature, oversimplifies the true nature of supercooled liquids
under shear flow. An understanding of the mechanism of isotropies and
anisotropies in the responses and fluctuations will lead to a better
appreciation of these violations of the FDT, as well as certain consequent
modifications to the concept of an effective temperature.Comment: 15pages, 17figure
Composite Double-Network Hydrogels To Improve Adhesion on Biological Surfaces.
Despite the development of hydrogels with high mechanical properties, insufficient adhesion between these materials and biological surfaces significantly limits their use in the biomedical field. By controlling toughening processes, we designed a composite double-network hydrogel with ∼90% water content, which creates a dissipative interface and robustly adheres to soft tissues such as cartilage and meniscus. A double-network matrix composed of covalently cross-linked poly(ethylene glycol) dimethacrylate and ionically cross-linked alginate was reinforced with nanofibrillated cellulose. No tissue surface modification was needed to obtain high adhesion properties of the developed hydrogel. Instead, mechanistic principles were used to control interfacial crack propagation. Comparing to commercial tissue adhesives, the integration of the dissipative polymeric network on the soft tissue surfaces allowed a significant increase in the adhesion strength, such as ∼130 kPa for articular cartilage. Our findings highlight the significant role of controlling hydrogel structure and dissipation processes for toughening the interface. This research provides a promising path to the development of highly adhesive hydrogels for tissues repair