Hydrodynamically tunable optofluidic cylindrical microlens{{ Xiaole Mao, ab John Robert Waldeisen, a Bala Krishna Juluri a and Tony Jun Huang* ab Received 12th June 2007, Accepted 12th July 2007 First published as an Advance Article on the web 2nd August 2007 DOI: 10.1039/b708863a In this work, we report the design, fabrication, and characterization of a tunable optofluidic microlens that focuses light within a microfluidic device. The microlens is generated by the interface of two co-injected miscible fluids of different refractive indices, a 5 M CaCl 2 solution (n D = 1.445) and deionized (DI) water (n D = 1.335). When the liquids flow through a 90-degree curve in a microchannel, a centrifugal effect causes the fluidic interface to be distorted and the CaCl 2 solution bows outwards into the DI water portion. The bowed fluidic interface, coupled with the refractive index contrast between the two fluids, yields a reliable cylindrical microlens. The optical characteristics of the microlens are governed by the shape of the fluidic interface, which can be altered by simply changing the flow rate. Higher flow rates generate a microlens with larger curvature and hence shorter focal length. The changing of microlens profile is studied using both computational fluid dynamics (CFD) and confocal microscopy. The focusing effect is experimentally characterized through intensity measurements and image analysis of the focused light beam, and the experimental data are further confirmed by the results from a ray-tracing optical simulation. Our investigation reveals a simple, robust, and effective mechanism for integrating optofluidic tunable microlenses in lab-on-a-chip systems. Introduction The motive to integrate tunable optical lenses within a microfluidic device stems from the promise of lab-on-a-chip systems. These systems are designed to perform a variety of on- chip biological/chemical assays, such as cell sorting, 1 single cell analysis, 2 and single molecule detection, 3 all of which will benefit from adaptive optical detection systems. While pumps, valves, and switches have been successfully integrated into lab-on-a-chip systems to allow flexible on-chip sample manipulation, 2,4 many critical components for optical detection still remain off-chip. 5 Therefore, in the past decade there has been tremendous interest in the development of on-chip adaptive optical components, especially tunable lenses. In-plane polydimethylsiloxane (PDMS) lenses have been demonstrated to increase the excitation light intensity in neighboring microfluidic channels and chambers during optical detection processes. 6–10 Although these lenses can be easily integrated within microfluidic devices, they share the common disadvantage that the geometries of these lenses are permanently fixed, as dictated by the fabrication process. Therefore, such lens designs lack the necessary mechanism to dynamically adjust focal length by reconfiguring the lens profile. Tunable lenses that alter the focal length through defor- mation of the lens profile offer significant advantages in terms of functionality and versatility. Due to its unlimited deformability, liquid is a desirable medium for tunable lenses. A commonly used technique is to seal liquid in a micro- chamber with an elastic membrane. The curvature of the membrane can be altered by hydraulic pressure 11–15 or by applying a mechanical actuation directly onto the mem- brane. 16 Another type of tunable liquid lens operates by manipulating the interface between immiscible fluids (i.e., water–oil or water–air). A variety of tuning mechanisms based on electrowetting, 17 stimuli-responsive hydrogels, 18 and redox surfactants 19 have been demonstrated. Unfortunately, complex fabrication procedures are often required to incorporate the tuning mechanism into these lenses. Furthermore, as most of these tunable lenses only allow focusing of light in the direction perpendicular to the device substrate, extra assembly steps are necessary to integrate these lenses with other microfluidic components. These disadvantages ultimately hinder the applicability of integrating the current tunable lenses into on-chip optical systems. The shortcomings of the above-mentioned lenses call for the development of microlenses that can be conveniently tuned, simply fabricated, and seamlessly integrated with other microfluidic components. Recent developments in optoflui- dics 20–25 (the combination of microfluidics and optics) have created prospective possibilities toward the obtainment of this goal. In optofluidic devices, optics can be built entirely out of liquids and one can easily change the optical properties of the device by manipulating fluid flows. 5 Such advantages enable fabrication and tuning of purely fluidic, adaptive optical components within microfluidic devices. An intriguing exam- ple of one such device is the liquid–liquid (L 2 ) waveguide, 20 whose entire structure was constructed using a sheath flow of CaCl 2 solution and DI water. Through the manipulation of the fluid flow rate, the core (CaCl 2 solution) and cladding (DI a Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA, 16802, USA. E-mail: junhuang@psu.edu; Fax: +1-814-865-9974; Tel: +1-814-863-4209 b Department of Bioengineering, The Pennsylvania State University, University Park, PA, 16802, USA { Electronic supplementary information (ESI) available: Additional simulation and experiment results. See DOI: 10.1039/b708863a { The HTML version of this article has been enhanced with colour images. PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2007 Lab Chip, 2007, 7, 1303–1308 | 1303