Optofluidics

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• Multiphysics of Microfluidics [ Acoustofluidics | Optofluidics ]
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Optofluidics refers to manipulation of light using fluids, or vice-verse, on the micro to nano meter scale. By taking advantage of the microfluidic manipulation, the optical properties of the fluids can be precisely and flexibly controlled to realize reconfigurable optical components which are otherwise difficult or impossible to implement with solid-state technology. In addition, the unique behavior of fluids on micro/nano scale has given rise to the possibility to manipulate the fluid using light.

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Go to Top | References

Lapsley, M. I. et al., Lab Chip, 2011, DOI:10.1039/c0lc00707b

We have developed a planar, optofluidic Mach–Zehnder interferometer for the label-free detection of liquid samples. In contrast to most on-chip interferometers which require complex fabrication, our design was realized via a simple, single-layer soft lithography fabrication process. In addition, a singlewavelength laser source and a silicon photodetector were the only optical equipment used for data collection. The device was calibrated using published data for the refractive index of calcium chloride (CaCl2) in solution, and the biosensing capabilities of the device were tested by detecting bovine serum albumin (BSA). Our design enables a refractometer with a low limit of detection (1.24 x 10^-4 refractive index units (RIU)), low variability (1 x 10^-4 RIU), and high sensitivity (927.88 oscillations per RIU). This performance is comparable to state-of-the-art optofluidic refractometers that involve complex fabrication processes and/or expensive, bulky optics. The advantages of our device (i.e. simple fabrication process, straightforward optical equipment, low cost, and high detection sensitivity) make it a promising candidate for future mass-producible, inexpensive, highly sensitive, label-free optical detection systems.

Figure: (a) Schematic of the device with arrows depicting the familiar light path of a Mach–Zehnder interferometer. (b) An experimental image of the device, post-insertion of optical fibers.

Mao, X., et al., Biomicrofluidics, 2010

We have designed, demonstrated, and characterized a simple, novel in-plane tunable optofluidic microlens. The microlens is realized by utilizing the interface properties between two different fluids: CaCl2 solution and air. A constant contact angle of 90° is the pivotal factor resulting in the outward bowing and convex shape of the CaCl2 solution-air interface. The contact angle at the CaCl2 solution-air interface is maintained by a flared structure in the polydimethylsiloxane channel. The resulting bowing interface, coupled with the refractive index difference between the two fluids, results in effective in-plane focusing. The versatility of such a design is confirmed by characterizing the intensity of a traced beam experimentally and comparing the observed focal points with those obtained via ray-tracing simulations. With the radius of curvature conveniently controlled via fluid injection, the resulting microlens has a readily tunable focal length. This ease of operation, outstandingly low fluid usage, large range tunable focal length, and in-plane focusing ability make this lens suitable for many potential lab-on-a-chip applications such as particle manipulation, flow cytometry, and in-plane optical trapping.

Figure: (a) Principle of the meniscus lens. The device includes a bowed structure. When the CaCl2 solution is injected, the fluid advances through the flange while the contact angle remains constant. The unchanged contact angle with the fluidic volume change results in a varied meniscus radius. The varied meniscus radius can be used to focus light. (b) A schematic of the lens, including an optical fiber, aperture (dark ink channel) to block the undesired light, bubble-tuning channel, and ray-tracing chamber that is filled with fluorescent dye for visualizing the focused beam profile.

Huang H., et al., Lab Chip, 2010, DOI:10.1039/c005071g

We report a two-dimensional (2D) tunable liquid gradient refractive index (L-GRIN) lens for variable focusing of light in the out-of-plane direction. This lens focuses a light beam through a liquid medium with a 2D hyperbolic secant (HS) refractive index gradient. The refractive index gradient is established in a microfluidic chamber through the diffusion between two fluids with different refractive indices, i.e. CaCl2 solution and deionized (DI) water. The 2D HS refractive index profile and subsequently the focal length of the L-GRIN lens can be tuned by changing the ratio of the flow rates of the CaCl2 solution and DI water. The focusing effect is experimentally characterized through side-view and top-view image analysis, and the experimental data match well with the results from ray-tracing optical simulations. Advantages of the 2D L-GRIN lens include simple device fabrication procedure, low fluid consumption rate, convenient lens-tuning mechanism, and compatibility with existing microfluidic devices. We expect that with further optimizations, this 2D L-GRIN lens can be used in many opticsbased lab-on-a-chip applications.

Figure: Principle of the 2D L-GRIN lens. (a) The 2D L-GRIN lens structure is composed of an L-GRIN lens chamber, two inlets for CaCl2 solution, four inlets for DI water, and two outlets. The diffusion of CaCl2 inside the L-GRIN lens chamber results in a 2D axis-symmetric hyperbolic secant (HS) refractive index profile in the X–Y plane, which can be used to focus the light beam along the Z direction. The lens is bonded to a glass substrate and a dye chamber is bonded on the opposite side of the glass substrate for visualization purpose. The input light is generated via a laser diode aligned vertically to the device plane. (b) The side view of the refractive index distribution in the L-GRIN lens chamber. The focal point of the 2D L-GRIN lens can be shifted along Z-axis (e.g., from b1 to b2 by adjusting the refractive index gradient within the liquid medium). © The fluid injection setup of the 2D L-GRIN lens (dye chamber is not included in this image). (d) A microscopic image of the 2D diffusion pattern in the lens chamber.

Mao, X. et al., Lab on a Chip, Vol. 9, pp. 2050-2058, 2009.

We report a tunable optofluidic microlens configuration named the Liquid Gradient Refractive Index (L-GRIN) lens for focusing light within a microfluidic device. The focusing of light was achieved through the gradient refractive index (GRIN) within the liquid medium, rather than via curved refractive lens surfaces. The diffusion of solute (CaCl2) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant refractive index profile to focus light. Tailoring the refractive index profile by adjusting the flow conditions enables not only tuning of the focal distance (translation mode), but also shifting of the output light direction (swing mode), a second degree of freedom that to our knowledge has yet to be accomplished for in-plane tunable microlenses. Advantages of the L-GRIN lens also include a low fluid consumption rate, competitive focusing performance, and high compatibility with existing microfluidic devices. This work provides a new strategy for developing integrative tunable microlenses for a variety of lab-on-a-chip applications.

Figure: Principle and design of the L-GRIN lens. (A) A schematic diagram showing the comparison between the classic refractive lens (A1) and GRIN lens (A2). Change of the refractive index contrast in GRIN lens can result in change of focal distance (A2–A3), and shift of optical axis can result in change of output light direction (A4). (B) Schematic of the L-GRIN lens design (B1), microscopic image of the L-GRIN lens in operation (B2, left), and the expected refractive index distribution at two locations (I and II) inside the lens (B2, right). High optical contrast areas (dark streaks) were observed near the fluidic boundaries (B2, left), suggesting significant variation of refractive index due to the CaCl2 diffusion. (C) Schematic drawing showing two operation modes of the L-GRIN lens: the translation mode with variable focal length including no-focusing (C1), a large focal distance (C2), and a small focal distance (C3); and the swing mode with variable output light direction (C3–C5).

Shi, J., et al., Microfluid Nanofluid DOI 10.1007/s10404-009-0548-9

We demonstrate a tunable in-plane optofluidic microlens with a 99 light intensity enhancement at the focal point. The microlens is formed by a combination of a tunable divergent air–liquid interface and a static polydimethylsiloxane lens, and is fabricated using standard soft lithography procedures. When liquid flows through a straight channel with a side opening (air reservoir) on the sidewall, the sealed air in the side opening bends into the liquid, forming an air–liquid interface. The curvature of this air–liquid interface can be conveniently and predictably controlled by adjusting the flow rate of the liquid stream in the straight channel. This change in the interface curvature generates a tunable divergence in the incident light beam, in turn tuning the overall focal length of the microlens. The tunability and performance of the lens are experimentally examined, and the experimental data match well with the results from a ray-tracing simulation. Our method features simple fabrication, easy operation, continuous and rapid tuning, and a large tunable range, making it an attractive option for use in lab-on-a-chip devices, particularly in microscopic imaging, cell sorting, and optical trapping/manipulating of microparticles.

Figure: a Schematic of the tunable lens. b–e Optical images of the device (region enclosed by the dotted square in a) taken at different flow rates of DI water in the microchannel: b 0 lL/min. c 20 lL/min. d 40 lL/min. e 60 lL/min. Liquid in the channel and monitor chamber is an aqueous solution of rhodamine fluorescent dye

Lapsley, M. I. et al., Appl. Phys. Lett., Vol. 95, pp. 083507, 2009.

We introduce an optofluidic based variable optical attenuator with high stability, high reliability, simple and inexpensive fabrication, and an attenuation performance comparable to commercial devices. A standard soft lithography process produces a single-layered polydimethylsiloxane (PDMS) microfluidic device integrated with optical fibers. By altering the refractive index of the fluid within the microchannel, we can control the reflectivity of the fluid/PDMS interface and thus achieve variable attenuation. Theoretical calculations are conducted based on Snell’s law of refraction and the Fresnel equations of reflection, and the calculated attenuation response matches well with experimental data.

Figure: A schematic of the optical attenuator device. Altering the flow rates Q1 and Q2 changes the reflected power (R) of the incident beam (I).

Mao, X. et al., Lab on a Chip, Vol. 7, pp. 1260-1262, 2007.

We introduce a novel fluid manipulation technique named “microfluidic drifting” to enable three-dimensional (3D) hydrodynamic focusing with a simple single-layer planar microfluidic device. Schematic of the 3D hydrodynamic focusing process by employing the ‘‘microfluidic drifting’’ technique. The 3D hydrodynamic focusing is accomplished in a two-step sequence. The first step focuses the sample flow in the vertical direction by using what we call the ‘‘microfluidic drifting’’ technique. This term refers to the lateral drift of the sample flow, caused by the transverse secondary flow induced by the centrifugal effect in the curve of a microfluidic channel. The secondary flow is characterized by a pair of counter-rotating vortices (Dean vortices) positioned in the upper and lower portion of the channel cross-sectional plane.

Figure: (Left) Slices are the cross-sectional profiles of the fluorescein dye concentration in the focusing device. (Center) The 3D architecture of the sample flow during the focusing process characterized by confocal microscopy and (Right) the CFD simulation performed under the same flow conditions.

Mao, X. et al., Lab on a Chip, Vol. 7, pp. 1303-1308, 2007.

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 CaCl2 solution (nD = 1.445) and deionized (DI) water (nD = 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 CaCl2 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.

Go to Top | Research Highlights

1. Michael Ian Lapsley, I-Kao Chiang, Yue Bing Zheng, Xiaoyun Ding, Xiaole Mao, and Tony Jun Huang, A Single-Layer, Planar, Optofluidic Mach-Zehnder Interferometer for Label-Free Detection, Lab on a Chip, 2011, DOI:10.1039/C0LC00707B. [PDF]
2. Xiaole Mao, Zackary I. Stratton, Ahmad Ahsan Nawaz, Sz-Chin Steven Lin, and Tony Jun Huang, Optofluidic Tunable Microlens by Manipulating the Liquid Meniscus Using a Flared Microfluidic Structure, Biomicrofluidics, Vol. 4, pp. 043007, 2010. [PDF]
3. Hua Huang, Xiaole Mao, Sz-Chin Steven Lin, Brian Kiraly, Yiping Huang, and Tony Jun Huang, Tunable Two-Dimensional Liquid Gradient Refractive Index (L-GRIN) Lens for Variable Light Focusing, Lab on a Chip, Vol. 10, pp. 2387-2393, 2010. (featured as back cover image) [PDF]
4. Jinjie Shi, Zak Stratton, Sz-Chin Steven Lin, Hua Huang and Tony Jun Huang, Tunable Optofluidic Microlens through Active Pressure Control of an Air-Liquid Interface, Microfluidics and Nanofluidics, Vol. 9, pp. 313-318, 2010. [PDF]
5. Xiaole Mao, Bala Krishna Juluri, Michael Ian Lapsley, Zackary S. Stratton and Tony Jun Huang, Milliseconds Microfluidic Chaotic Bubble Mixer, Microfluidics and Nanofluidics, Vol. 8, pp. 139-144, 2010. [PDF]
6. Michael Ian Lapsley, Sz-Chin Steven Lin, Xiaole Mao and Tony Jun Huang, An In-Plane, Variable Optical Attenuator Using a Fluid-Based Tunable Reflective Interface, Applied Physics Letters, Vol. 95, pp. 083507, 2009. [PDF]
7. Xiaole Mao, Sz-Chin Steven Lin, Michael Ian Lapsley, Jinjie Shi, Bala Krishna Juluri, and Tony Jun Huang, Tunable Liquid Gradient Refractive Index (L-GRIN) Lens with Two Degrees of Freedom, Lab on a Chip, Vol. 9, pp. 2050-2058, 2009. [PDF]
8. Xiaole Mao, Sz-Chin Steven Lin, Cheng Dong, and Tony Jun Huang, Single-Layer Planar On-Chip Flow Cytometer Using Microfluidic Drifting Based Three-Dimensional (3D) Hydrodynamic Focusing, Lab on a Chip, Vol. 9, pp. 1583-1589, 2009. [PDF]
9. Xiaole Mao, Tony Jun Huang, Focusing Fluids and Light in Micro/Nano Scale – Enabling Technologies for Single-Particle Detection, IEEE Nanotechnology Magazine, Vol. 2, pp. 22-27, 2008. [PDF]
10. Xiaole Mao, John Robert Waldeisen, Bala Krishna Juluri, Tony Jun Huang, Hydrodynamically Tunable Optofluidic Cylindrical Microlens, Lab on a Chip, Vol. 7, pp. 1303-1308, 2007. [PDF]
11. Xiaole Mao, John Robert Waldeisen, Tony Jun Huang, “Microfluidic Drifting” - Implementing Three-Dimensional Hydrodynamic Focusing with a Single-Layer Planar Microfluidic Device, Lab on a Chip, Vol. 7, pp. 1260-1262, 2007. (featured as front cover image) [PDF]