Generation of Aspherical Optical Lenses via Arrested Spreading and Pinching of a Cross-Linkable Liquid.

Aspherical optical lenses with spatially varying curvature are desired for capturing high quality, aberration free images in numerous optical applications. Conventionally such lenses are prepared by multistep top-down processes which are expensive, time-consuming, and prone to high failure rate. In this context, an alternate method is presented here based on arrested spreading of a sessile drop of a transparent, cross-linkable polymeric liquid on a solid substrate heated to an elevated temperature. Whereas surface tension driven flow tends to render it spherical, rapid cross-linking arrests such flow so that nonequilibrium aspherical shapes are attained. It is possible to tune also the initial state of the drop via delayed pinching of a liquid cylinder which precedes its release on the substrate. This method has led to the generation of a wide variety of optical lenses, ranging from spherical plano convex to superspherical solid immersion to exotic lenses not achieved via conventional methods.

Control of adhesion via internally pressurized subsurface microchannels.

While pressure sensitive adhesives in general consist of a layer of viscoelastic glue sandwiched between two adherents, we explore here the design of an adhesive embedded with microchannels which remain either open to atmosphere or pressurized to different positive and negative pressures. We subject these layers to indentation by a rigid cylinder such that in addition to adhesion between the indenter and the adhesive surface, the inner walls of the channels too self-adhere; during retraction of the indenter, these surfaces debond, but at a different load, thus resulting in hysteresis. When these channels are pressurized to different extents, the contact areas of various interfaces vary, so also the resultant hysteresis. For experiments with constant depth of indentation, the hysteresis increases and attains maxima at an intermediate value of the internal pressure inside the channels. The hysteresis increases also with the skin thickness of the adhesive over the channels. These results show that subsurface channels in an adhesive allow active manipulation of adhesion over a large range via coupled effect of geometry of channels, their surface characteristics, and the pressure inside.

Bioinspired design of a hierarchically structured adhesive.

The mechanism by which many creatures such as geckos can run at ease on a vertical wall and yet remain strongly adhered has been linked to hierarchically patterned microstructures: flexible pads, hairs, and subsurface fluidic vessels at their feet. Despite many advances, how these features of different length scales and the associated physical phenomena couple to engender this "smart" adhesive is yet to be understood and mimicked. In this context, we have designed elastomeric films of poly(dimethylsiloxane) embedded with stacks of planar microchannels, curved and straight, and channels with microscopically patterned walls. We have altered also chemically the adhesive surface including that of the microchannel walls by creating dangling chains. During indentation experiments, deformation and self-adhesion of these structures enhance the effective area of adhesion with a consequent increase in adhesion hysteresis over orders of magnitude. In addition, suitable orientation of these buried channels allows the generation of load dependent hysteresis and its spatial modulation.