Accurate determination of fractional retardations in photoelasticity through digital fringe multiplication.

Digital fringe multiplication is a useful technique for obtaining fractional fringe orders in photoelasticity at regions with very feeble retardations. In this paper, the effectiveness of the existing fringe multiplication techniques is investigated theoretically and experimentally. A new approach for fringe multiplication, using background nullified fringes, is developed to overcome the prevailing issues. Three options for obtaining background nullified fringe patterns are explained. Further, quadrature transformation of these fringe patterns is carried out to fetch uniform modulation. Moreover, a simplified fringe multiplication method is worked out to obtain fractional fringes from the uniformly modulated fringe patterns. The proposed procedures have been demonstrated through simulated as well as experimental images. A parametric study is carried out to understand the influence of pixel resolution and bit depth of the images on fringe multiplication. A criterion is established for finding out the maximum possible fringe multiplication for a given initial pixel/fringe resolution. It is observed that, for higher levels of fringe multiplication, larger bit depth of the image in conjunction with sufficient fringe resolution would be essential. The same also holds for fringe multiplication at areas with stress concentrations.

Electrowetting actuated microfluidic transport in surface grooves with triangular cross section.

Liquids show different static wetting morphologies in open triangular grooves depending upon the wedge angle (ψ) of the groove and the liquid contact angle (θ) with the substrate. Switching between different morphologies can be achieved either by varying the contact angle of the liquid or by changing the wedge angle of the groove. In the present work we manipulate the apparent contact angle of a liquid by electrowetting to switch between liquid morphologies, from droplet to filament, to achieve microfluidic transport of the liquid into open triangular grooves. The static length of liquid filaments in grooves is analyzed as a function of applied voltage for different applied ac frequencies. The dynamic advancement of the filament lengths in grooves is analyzed as a function of time for different applied voltages for two different liquids: first with contact angle greater than the wedge angle and second with contact angle smaller than the wedge angle. Later an exact electrical model is derived to explain the liquid transport in triangular grooves actuated by electrowetting which includes the precise geometry of the liquid morphology.

Interfacial growth during closure of a cutaneous wound: stress generation and wrinkle formation.

A biomechanical growth model for the proliferation stage of cutaneous wound healing is developed emphasizing the emergence of stress and wrinkled skin during the healing process. The healing is assumed to be primarily driven by growth at the wound edge (i.e. the interface between the wound and the skin) leading to incompatible growth strains. A closed form solution of the boundary value problem is obtained using a Varga hyperelastic membrane model for both the skin and the wound. The nature of the solution is explored for various parametric values of the skin tension, healing rate, edge incompatibility, wrinkled region radius, and wound stiffness. The obtained results for the stress field, wrinkling, and rate of healing are qualitatively in good agreement with the existing experimental observations.

Mechanics of cutaneous wound rupture.

A cutaneous wound may rupture during healing as a result of stretching in the skin and incompatibility at the wound-skin interface, among other factors. By treating both wound and skin as hyperelastic membranes, and using a biomechanical framework of interfacial growth, we study rupturing as a problem of cavitation in nonlinear elastic materials. We obtain analytical solutions for deformation and residual stress field in the skin-wound configuration while emphasizing the coupling between wound rupture and wrinkling in the skin. The solutions are analyzed in detail for variations in stretching environment, healing condition, and membrane stiffness.