Adjustable hybrid diffractive/refractive achromatic lens.

We demonstrate a variable focal length achromatic lens that consists of a flat liquid crystal diffractive lens and a pressure-controlled fluidic refractive lens. The diffractive lens is composed of a flat binary Fresnel zone structure and a thin liquid crystal layer, producing high efficiency and millisecond switching times while applying a low ac voltage input. The focusing power of the diffractive lens is adjusted by electrically modifying the sub-zones and re-establishing phase wrapping points. The refractive lens includes a fluid chamber with a flat glass surface and an opposing elastic polydimethylsiloxane (PDMS) membrane surface. Inserting fluid volume through a pump system into the clear aperture region alters the membrane curvature and adjusts the refractive lens' focal position. Primary chromatic aberration is remarkably reduced through the coupling of the fluidic and diffractive lenses at selected focal lengths. Potential applications include miniature color imaging systems, medical and ophthalmic devices, or any design that utilizes variable focal length achromats.

In vitro fracture testing of submicron diameter collagen fibril specimens.

Mechanical testing of collagenous tissues at different length scales will provide improved understanding of the mechanical behavior of structures such as skin, tendon, and bone, and also guide the development of multiscale mechanical models. Using a microelectromechanical-systems (MEMS) platform, stress-strain response curves up to failure of type I collagen fibril specimens isolated from the dermis of sea cucumbers were obtained in vitro. A majority of the fibril specimens showed brittle fracture. Some displayed linear behavior up to failure, while others displayed some nonlinearity. The fibril specimens showed an elastic modulus of 470 ± 410 MPa, a fracture strength of 230 ± 160 MPa, and a fracture strain of 80% ± 44%. The fibril specimens displayed significantly lower elastic modulus in vitro than previously measured in air. Fracture strength/strain obtained in vitro and in air are both significantly larger than those obtained in vacuo, indicating that the difference arises from the lack of intrafibrillar water molecules produced by vacuum drying. Furthermore, fracture strength/strain of fibril specimens were different from those reported for collagenous tissues of higher hierarchical levels, indicating the importance of obtaining these properties at the fibrillar level for multiscale modeling.

Tunable-focus flat liquid-crystal diffractive lens.

We demonstrate an innovative variable-focus flat liquid-crystal diffractive lens (LCDL) with 95% diffraction efficiency and millisecond switching times using a +/-2.4 V ac input. This lens is based on the electrical modulation of a 3 mum layer of nematic liquid-crystal sandwiched between a Fresnel zone electrode structure and a reference substrate. Each zone is divided into 12 subzones to digitize the phase profiles and define the phase wrapping points. The focusing power can rapidly be switched by electrically changing the number of subzones and re-establishing the wrapping points. Potential applications include zooms with no moving parts and autofocus lenses for compact imaging devices.

Stress-strain experiments on individual collagen fibrils.

Collagen, a molecule consisting of three braided protein helices, is the primary building block of many biological tissues including bone, tendon, cartilage, and skin. Staggered arrays of collagen molecules form fibrils, which arrange into higher-ordered structures such as fibers and fascicles. Because collagen plays a crucial role in determining the mechanical properties of these tissues, significant theoretical research is directed toward developing models of the stiffness, strength, and toughness of collagen molecules and fibrils. Experimental data to guide the development of these models, however, are sparse and limited to small strain response. Using a microelectromechanical systems platform to test partially hydrated collagen fibrils under uniaxial tension, we obtained quantitative, reproducible mechanical measurements of the stress-strain curve of type I collagen fibrils, with diameters ranging from 150-470 nm. The fibrils showed a small strain (epsilon < 0.09) modulus of 0.86 +/- 0.45 GPa. Fibrils tested to strains as high as 100% demonstrated strain softening (sigma(yield) = 0.22 +/- 0.14 GPa; epsilon(yield) = 0.21 +/- 0.13) and strain hardening, time-dependent recoverable residual strain, dehydration-induced embrittlement, and susceptibility to cyclic fatigue. The results suggest that the stress-strain behavior of collagen fibrils is dictated by global characteristic dimensions as well as internal structure.

Light scattering investigation above the nematic-smectic-A phase transition in binary mixtures of calamitic and bent-core mesogens.

Quasielastic light scattering measurements were performed in the nematic phase of mixtures consisting of the calamitic mesogen 8OCB doped with small concentrations of the bent-core molecule P-7PIMB. It was found that the regular part of the bend elastic constant decreases strongly with dopant concentration X. Close to the nematic-smectic-A phase transition temperature, the divergent part of the bend elastic constant, which is proportional to the bare correlation length xi(0)(||) parallel to the layer normal, also decreases rapidly with X. The effect of the dopant on xi(0)(||) is examined in brief theoretically.

Stability of connected cylindrical liquid bridges.

Two cylindrical liquid bridges, with a conduit to facilitate flow of liquid from one bridge to the other, were levitated against gravity in a magnetic field gradient. The stability limit of the bridges subjected to near zero total body force was measured as a function of their slenderness ratios, and found to be in good agreement with theoretical predictions.