Dynamic interface pressure distributions of two transtibial prosthetic socket concepts.

In this study, we investigated and compared the dynamic interface pressure distribution of hands-off and hands-on transtibial prosthetic systems by means of pressure mapping. Of the 48 established unilateral amputees recruited, half (n = 24) had been wearing pressure-cast prostheses (IceCast Compact) and the other half (n = 24) had been wearing hand-cast sockets of the patellar tendon bearing design. We measured the dynamic pressure profile of more than 90% of the area within each prosthetic socket by means of four Tekscan F-Scan socket transducer arrays. We compared the interface pressure between socket concepts. We found that the distribution of dynamic pressure at the limb-socket interface was similar for the two intervention (socket prescription) groups. However, a significant difference was found in the magnitude of the interface pressure between the two socket concepts; the interface pressures recorded in the hands-off sockets were higher than those seen in the hands-on concept. Despite the differences in interface pressure, the level of satisfaction with the sockets was similar between subject groups. The sockets instrumented for this study had been in daily use for at least 6 months, with no residual-limb health problems.

Effect of decoupled azimuthal and zenithal anchoring on smectic-C chevron structures.

We present a study of the effect of decoupled azimuthal and zenithal weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. We consider temperatures below the SmA-SmC transition and assume that the value of the smectic cone angle can be regarded as constant through the cell. By standard Euler-Lagrange minimization of the total energy we obtain a simple analytical expression for the equilibrium director twist angle in the C1 and C2 chevron states. Using this analytical form, we are able to compare the total energies of the C1 and C2 chevrons, and determine the globally stable chevron profile. We show that the C2 state is preferred when the azimuthal anchoring strength is relatively large, while C1 chevrons will dominate for strong zenithal anchoring.

Mechanical actions on nanocylinders in nematic liquid crystals.

We apply the Landau-de Gennes theory to study the equilibrium problem that arises when a cylinder of radius R is kept at a given distance h from a plane wall. We assume that both the lateral boundary of the cylinder and the wall enforce homeotropic anchoring conditions on the liquid crystal, which prescribe the liquid crystal molecules to stick orthogonally to the bounding surfaces. Typically, in our study R ranges from a few to hundreds of biaxial coherence lengths, where a biaxial coherence length, which depends on the temperature, is a few nanometers. The equilibrium textures exhibit a bifurcation between a flat solution, where one eigenvector of the order tensor Q is everywhere parallel to the cylinder's axis, and an escape solution, where the eigenframe of Q flips out of the plane orthogonal to the cylinder's axis. The escape texture minimizes an appropriately renormalized energy functional F(*) for h>h(c), while the flat texture minimizes F(*) for h< h(c). We compute both the force and the torque transmitted to the cylinder by the surrounding liquid crystal and we find that the diagrams of both as functions of h fail to be monotonic along the escape texture. Thus, upon decreasing h, a snapping instability is predicted to occur, with an associated hysteresis loop in the force diagram, before h reaches h(c). Finally, since the symmetry of this problem makes it equivalent to the one where two parallel cylinders are separated by the distance 2h , the snapping instability predicted here should also be observed there.