We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based acoustic streaming. The device operates by trapping an air bubble within a "horse-shoe" structure located between two laminar flows inside a microchannel. Acoustic waves excite the trapped air bubble at its resonance frequency, resulting in acoustic streaming, which disrupts the laminar flows and triggers the two fluids to mix. Due to this technique's simple design, excellent mixing performance, and fast mixing speed (a few milliseconds), our single-bubble-based acoustic micromixer may prove useful for many biochemical studies and applications.

A gold nanodisk array, coated with bistable, redox-controllable [2]rotaxane molecules, when exposed to chemical oxidants and reductants, undergoes switching of its plasmonic properties reversibly. By contrast, (i) bare gold nanodisks and (ii) disks coated with a redox-active, but mechanically inert, control compound do not display surface-plasmon-based switching. Along with calculations based on time-dependent density functional theory, these experimental observations suggest that the nanoscale movements within surface-bound "molecular machines" can be used as the active components in plasmonic devices.

A microcantilever, coated with a monolayer of redox-controllable, bistable [3]rotaxane molecules (artificial molecular muscles), undergoes reversible deflections when subjected to alternating oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol-gold chemistry. An electrochemical cell was employed in the experiments, and deflections were monitored both as a function of (i) the scan rate (< or =20 mV s(-1)) and (ii) the time for potential step experiments at oxidizing (>+0.4 V) and reducing (<+0.2 V) potentials. The different directions and magnitudes of the deflections for the microcantilevers, which were coated with artificial molecular muscles, were compared with (i) data from nominally bare microcantilevers precoated with gold and (ii) those coated with two types of control compounds, namely, dumbbell molecules to simulate the redox activity of the palindromic bistable [3]rotaxane molecules and inactive 1-dodecanethiol molecules. The comparisons demonstrate that the artificial molecular muscles are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. The approximately 550 nm deflections were calculated to be commensurate with forces per molecule of approximately 650 pN. The thermal relaxation that characterizes the device's deflection is consistent with the double bistability associated with the palindromic [3]rotaxane and reflects a metastable contracted state. The use of the cooperative forces generated by these self-assembled, nanometer-scale artificial molecular muscles that are electrically wired to an external power supply constitutes a seminal step toward molecular-machine-based nanoelectromechanical systems (NEMS).

Due to the low Reynolds number associated with microscale fluid flow, it is difficult to rapidly and homogenously mix two fluids. In this letter, we report a fast and homogenized mixing device through the use of a bubble-based microfluidic structure. This micromixing device worked by trapping air bubbles within the predesigned grooves on the sidewalls of the channel. When acoustically driven, the membranes (liquid/air interfaces) of these trapped bubbles started to oscillate. The bubble oscillation resulted in a microstreaming phenomenonstrong pressure and velocity fluctuations in the bulk liquid, thus giving rise to fast and homogenized mixing of two side-by-side flowing fluids. The performance of the mixer was characterized by mixing deionized water and ink at different flow rates. The mixing time was measured to be as small as 120 ms.

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.

Ordered Au nanodisk arrays were fabricated on glass substrates using nanosphere lithography combined with a two-step reactive ion etching technique. The optical properties of these arrays were investigated both experimentally and theoretically. Specifically, the effects of disk diameter on localized surface plasmon resonance ͑LSPR͒ were characterized and compared with results from discrete dipole approximation ͑DDA͒ calculations. The effects of glass substrate, Cr interfacial layer, and Au thickness on LSPR were investigated computationally. Furthermore, thermal treatment was found to be essential in improving the nanodisk arrays' LSPR properties. Using atomic force microscopy and DDA calculations, it was established that the improvements in LSPR properties were due to thermally induced morphologic changes. Finally, microfluidic channels were integrated with the annealed disk arrays to study the sensitivity of LSPR to the change in surroundings' refractive index. The dependence of LSPR on surroundings' refractive index was measured and compared with calculated results.

An optically controlled plasmonic switch that uses Au nanodisk arrays embedded in azobenzene‐doped, photoresponsive liquid crystals (LCs) is reported. The switch utilizes the photo‐induced phase transition of azobenzene‐doped LCs to alter the localized surface plasmon resonance (LSPR) of the Au nanodisks. The experimental modulation of the LSPR matches well with theoretical calculations. The influence of the angle of the incident light, power of the pump light, and wavelength of the probe light are also investigated.

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 (CaCl(2)) between side-by-side co-injected microfluidic laminar flows was utilized to establish a hyperbolic secant (HS) 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.