© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 874 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.de curved membrane enclosure. [20–23] Importantly, internal pres- sure within an enclosed membrane leads to a spherical lens, whereas, here, the wetting induced elastocapillary effect leads to aspherical geometry. [19] This particular effect differentiates our system from other conventional optofluidic lenses in which a curved liquid meniscus or a liquid drop is used as a lens, [24] and which essentially remains spherical. The dual effect of difference in refractive indices (r.i.) of liquid (ca. 1.33–1.52) and solid (r.i. ca. 1.41) and the curved interfaces in the layer results in a lensing effect. The focal length of these lenses can be tuned by number of geometric, material, and interfacial properties. The layers that house the lenses can be bonded to a rigid substrate, but can also be gen- erated on a flexible one, which allows their surface profile and resultant optical properties to be altered further via axial com- pression of the substrate. The lenses can be used also as optical filters, which thus allows compact design of optical elements in many applications. Figure 1a shows these lenses prepared in elastomeric layers of poly(dimethylsiloxane) (PDMS) bonded to a rigid glass slide. Layers of uniform thickness were prepared by cross- linking Sylgard 184 between two parallel plates that were separated by spacers of desired thickness ca. 0.5–1.5 mm. [17] The layers were embedded with channels at different vertical heights: t min ≈ 5–160 μm, by using cylindrical rods of diameter d ≈ 0.45–1.2 mm as templates. [17–19] These channels remain circular when filled with air; the elastomeric layer too remains undeformed, but when filled with a wetting liquid, e.g., silicone oil (surface tension, γ ≈ 22 mJ/m 2 ), excess interfacial energy of wetting is converted into elastic energy of the channel wall, which bulges out [18] at the region of minimum thickness, t min , as depicted by the side view in Figure 1b. The channel cross- section deforms from being circular to oval-shaped and remains symmetric about the location of t min . Over time the bulging height increases further because of diffusion of the liquid, how- ever small, into the solid network; finally a definite time-invar- iant profile is attained in about 24 h. Thus, the smooth surface of a soft elastomer becomes undulating with a spatially varying curvature which can be used as an aspherical lens. When light is incident upon the curved elastomer, a line of focused light is generated, the magnified image of which, when captured by using a camera fitted with a microscope lens, shows sharp edges surrounding the line (Figure 1a). Figure 1f shows curvature κ as a function of distance x from the location of t min for channels of constant diameter d = 450 μm but different minimum skin thickness t min = 20–98 μm. For all cases, κ remains positive in the vicinity of t min , but decreases away from it, eventually turning negative and then again posi- tive, and finally asymptotically converging to zero far away from Design of an Adaptable Optofluidic Aspherical Lens by Using the Elastocapillary Effect Abhijit Chandra Roy and Animangsu Ghatak* Dr. A. C. Roy, Dr. A. Ghatak Department of Chemical Engineering Indian Institute of Technology Kanpur 208016, India E-mail: aghatak@iitk.ac.in DOI: 10.1002/adom.201400081 Aspherical optical lenses comprise curved surfaces, the cur- vature of which varies spatially to generate a sharp image of an object that is devoid of common optical aberrations. These lenses are used extensively in a variety of applications: optical metrology, [1] spectroscopy, [2] laser diodes, [3] acousto-optics, [4] holographic gratings, [5] grating sensors, [6] and laser physics. [7] The lenses are used also in applications which require light to be focused in two dimensions, e.g., light-sheet microscopy, [8] line-illumination microscopy, [9] and depth-of-focus enhance- ment. [10] These applications all demand minimum optical aber- ration, a sharp line of focus, and also compact lens modules for ease of operation. [11] Conventionally, however, fabrication of aspherical lenses is a problem as they involve intricate design of a surface that precisely follows a specific profile. [12] The top- down approach to fabricating such surfaces involves machining and polishing a curved solid by using high-precision turning machines, operation of which requires a highly skilled techni- cian as well as precise control of temperature, humidity, and vibration in the surroundings; [13–15] all of these increase the manufacturing cost. The rate of rejection of faulty pieces also remains very high. The other strategy of generating aspherical profiles is to draw a preformed material at elevated temperature to form a cylindrical lens of desired curvature. [16] Here too, the starting material must be of a desired shape, and it is expected to sustain high temperature. The drawing process tends to gen- erate a spherical surface rather than an aspherical one, because of the effect of surface tension. Essentially, there exists no simple way of manufacturing aspherical lenses, which has a cascading effect on research in many different fields. We present a method of generating aspherical lenses by using an elastocapillary instability induced by surface tension of a soft rubbery solid. In particular, we have prepared thin elastomeric layers embedded with microchannels of different shapes and size of cross-section. When these channels are filled with a wetting liquid, interfacial energy is released, which bulges out the portion of the film at the vicinity of the channel, forming a surface with spatially varying curvature. [17–19] Thus, in contrast to conventional optofluidic lenses, [20,21] the bulging deformation of the elastomeric film occurs spontaneously without any fluidic pressure being exerted to alter the cur- vature of an enclosed membrane, nor the need for a prefixed Adv. Optical Mater. 2014, 2, 874–878