Thermocapillary actuation of liquid plugs using a heater array

With the aim toward realizing polymerase chain reaction (PCR) of deoxyribonucleic acid (DNA) in plug-based capillary platforms, this paper reports the theoretical and experimental results of thermocapillary actuation for temperature cycling with an arbitrary ramping function. Two concepts were investigated: (a) actuation and spatial temperature cycling with three heaters and (b) actuation and temporal cycling with two heaters. The paper first describes the analytical models of both concepts. The model considers the spatio-temporal heat transfer effects, which is coupled with the surface tension driven movement of the plug. In the experiments, both temperature field and plug motion were measured and evaluated. The temperature field was captured by an infrared thermal tracer camera. The position of the plugs was automatically captured and evaluated with a CCD camera. Finally, analytical and experimental results are compared and discussed.     

Dynamics of droplet transport induced by electrowetting actuation

This study reports on the dynamics of droplets in the capillary regime induced by electrowetting-on-dielectric actuation. The configuration investigated allows for comparing the experimental results with respect to the predictions of Brochard’s theoretical model (Brochard in Langmuir 5:432–438, 1989). Firstly, side-view observations using stroboscopic recording techniques were used to measure and analyse droplet deformations as well as the front and rear apparent contact angles during motion. Secondly, the influence of viscosity on the droplet velocity as a function of the applied voltage was studied. This has revealed that low Reynolds number droplet motion can be described by the simple laminar viscous model of Brochard. Finally, the influence of the dielectric thickness on the droplet dynamics was studied. It is shown that droplet velocity is limited by a saturation effect of the driving electrostatic force and that this phenomenon is very similar to that occurring in static experiments.     

Microfluidic mixing via transverse electrokinetic effects in a planar microchannel

A new micromixer incorporating integrated electrodes deposited on the bottom surface of a glass/PDMS microchannel is used to induce a localized, perpendicular electric field within pressure driven axial flow. The presence of the electric field drives electro-osmotic flow in the transverse direction along the channel walls, creating helical motion that serves to mix the fluid. A numerical model is used to describe the three-dimensional flow field, where characterization is performed via particle tracking of passive tracer particles, and the conditional entropy (S _lc) is utilized to approximate the extent of mixing along cross-sectional planes. The geometrical parameters and operating conditions of the numerical model are used to fabricate an experimental device, and fluorescence microscopy measurements are used to verify mixing of rhodamine B across the width of the microchannel for a wide range of fluid flow rates. The results demonstrate that under certain operating conditions and selective placement of the electrode gaps along the width of the microchannel, efficient mixing can be achieved within 6 mm of the inlet.

Thermocapillary actuation of droplets on chemically patterned surfaces by programmable microheater arrays

We have designed a microfluidic device for the actuation of liquid droplets or continuous streams on a solid surface by means of integrated microheater arrays. The microheaters provide control of the surface temperature distribution with high spatial resolution. These temperature gradients locally alter the surface tension along droplets and thin films thus propelling the liquid toward the colder regions. In combination with liquophilic and liquophobic chemical surface patterning, this device can be used as a logistic platform for the parallel and automated routing, mixing and reacting of a multitude of liquid samples, including alkanes, poly(ethylene glycol) and water.     

Electrowetting droplet microfluidics on a single planar surface

Electrowetting refers to an electrostatically induced reduction in the contact angle of an electrically conductive liquid droplet on a surface. Most designs ground the droplet by either sandwiching the droplet with a grounding plate on top or by inserting a wire into the droplet. Washizu and others have developed systems capable of generating droplet motion without a top plate while allowing the droplet potential to float. In contrast to these designs, we demonstrate an electrowetting system in which the droplet can be electrically grounded from below using thin conductive lines on top of the dielectric layer. This alternative method of electrically grounding the droplet, which we refer to as grounding-from-below, enables more robust droplet translation without requiring a top plate or wire. We present a concise electrical-energy analysis that accurately describes the distinction between grounded and non-grounded designs, the improvements in droplet motion, and the simplified control strategy associated with grounding-from-below designs. Electrowetting on a single planar surface offers flexibility for interfacing to liquid-handling instruments, utilizing droplet inertial dynamics to achieve enhanced mixing of two droplets upon coalescence, and increasing droplet translation speeds. In this paper, we present experimental results and a number of design issues associated with the grounding-from-below approach.     

Copper planar microcoils applied to magnetic actuation

Recent advances in microtechnology allow realization of planar microcoils. These components are integrated in MEMS as magnetic sensor or actuator. In the latter case, it is necessary to maximize the effective magnetic field which is proportional to the current passing through the copper track and depends on the distance to the generation microcoil. The aim of this work was to determine the optimal microcoil design configuration for magnetic field generation. The results were applied to magnetic actuation, taking into account technological constraints. In particular, we have considered different realistic configurations that involve a magnetically actuated device coupled to a microcoil. Calculations by a semi-analytical method using Matlab software were validated by experimental measurements. The copper planar microcoils are fabricated by UV micromoulding on different substrates: flexible polymer (Kapton^®) and silicate on silicon. They are constituted by a spiral-like continuous track. Their total surface is about 1 mm².     

Mixing and hydrodynamic analysis of a droplet in a planar serpentine micromixer

In this work we combined numerical simulation with molecular-diffusion effect, high-tempo micro-particle image velocimetry (μ-PIV), and probability distribution function (PDF) analysis to investigate the chaotic mixing and hydrodynamics inside a droplet moving through a planar serpentine micromixer (PSM). Robust solutions for the distributions of interface and concentration of the droplets were obtained via computational fluid dynamics. The simulated fluid patterns are consistent with those measured with μ-PIV, which serves as a powerful nonintrusive diagnostic approach to observe the droplets. Two mechanisms are proposed to enhance the performance of mixing in a PSM—the deformation of droplets and the asymmetric recirculation within the droplets. On introducing alternating cross sections into a winding channel, this specific design of PSM is found to amplify the fluid disturbance and maximum vorticity difference. Data show that the PDF of the vorticity fields is modified and the fraction with larger vorticity is increased. Accordingly, the PSM is capable of achieving a mixing index 90% within 700 μm (Re = 2), which is eight times better than for a straight microchannel. The results not only demonstrate explicitly the fluid patterns within the droplets but also provide significant insight into the factors dominating the mixing efficiency.     

Dynamic formulation of a planar 3-DOF parallel manipulator with actuation redundancy

In this paper, based on the conventional Newton–Euler approach, a simplification method is proposed to derive the dynamic formulation of a planar 3-DOF parallel manipulator with actuation redundancy. Closed-form solutions are developed for the inverse kinematics. Based on the kinematics, the Newton–Euler approach in simplification form is used to derive the inverse dynamic model of the redundant parallel manipulator. Then, the driving force optimization is performed by minimizing an objective function which is the square of the sum of four driving forces. The dynamic simulations are done for the parallel manipulator with both the redundant and non-redundant actuations. The result shows that the dynamic characteristics of the manipulator in the redundant case are better than that in the non-redundancy. The redundantly actuated parallel manipulator was incorporated into a 4-DOF hybrid machine tool which includes a feed worktable.     

Effects of topological changes in microchannel geometries on the asymmetric breakup of a droplet

Passive asymmetric breakups of a droplet could be done in many microchannels of various geometries. In order to study the effects of different geometries on the asymmetric breakup of a droplet, four types of asymmetric microchannels with the topological equivalence of geometry are designed, which are T-90, Y-120, Y-150, and I-180 microchannels. A three-dimensional volume of fluid multiphase model is employed to investigate the asymmetric rheological behaviors of a droplet numerically. Three regimes of rheological behaviors as a function of the capillary numbers Ca and the asymmetries As defined by As = (b1 ? b2)/(b1 + b2) (where b1 and b2 are the widths of two asymmetric sidearms) have been observed. A power law model based on three major factors (Ca, As and the initial volume ratio r ₀) is employed to describe the volume ratio of two daughter droplets. The analysis of pressure fields shows that the pressure gradient inside the droplet is one of the major factors causing the droplet translation during its asymmetric breakup. Besides the above similarities among various microchannels, the asymmetric breakup in them also have some slight differences as various geometries have different enhancement or constraint effects on the translation of the droplet and the cutting action of flows. It is disclosed that I-180 microchannel has the smallest critical capillary number, the shortest splitting time, and is hardest to generate satellite droplets.     

Numerical simulation of emergence of a water droplet from a pore into a microchannel gas stream

A numerical investigation on the dynamic behavior of liquid water entering a microchannel through a lateral opening (pore) in the wall is reported in this paper. The channel dimensions, flow conditions and transport properties are chosen to simulate those in the gas channel of a typical proton exchange membrane fuel cell (PEMFC). Two-dimensional transient simulations employing the volume of fluid method are used to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to airflow in the bulk of the microchannel. A series of parametric studies, including the effects of static contact angle, dimensions of the pore, air-inlet velocity, and water-inlet velocity are performed with a particular focus on the effect of hydrophobicity. The simulations show that the wettability of the microchannel surface has a major impact on the dynamics of the water droplet. Flow patterns are presented and analyzed showing the splitting of a droplet for a hydrophobic surface, and the tendency for spreading and film flow formation for a hydrophilic surface. The time evolution of the advancing and receding contact angles of the droplet are found to be sensitive to the wettability when the gas diffusion layer surface is hydrophilic, but independent of wettability when the surface is hydrophobic. The critical air velocity at which a droplet detaches is found to decrease with increasing hydrophobicity and with increasing initial dimension of the droplet. The critical air velocity found in the present study by taking into account the water transport and evolution of the droplet from a pore are found to differ significantly from previous works which consider a stagnant droplet sitting on the surface.     

Robust nonlinear control of a planar 2-DOF parallel manipulator with redundant actuation

A new robust nonlinear controller is presented and applied to a planar 2-DOF parallel manipulator with redundant actuation. The robust nonlinear controller is designed by combining the nonlinear PD (NPD) control with the robust dynamics compensation. The NPD control is used to eliminate the trajectory disturbances, unmodeled dynamics and nonlinear friction, and the robust control is used to restrain the model uncertainties of the parallel manipulator. The proposed controller is proven to guarantee the uniform ultimate boundedness of the closed-loop system by the Lyapunov theory. The trajectory tracking experiment with the robust nonlinear controller is implemented on an actual planar 2-DOF parallel manipulator with redundant actuation. The experimental results are compared with the augmented PD (APD) controller, and the proposed controller shows much better trajectory tracking accuracy.     

Bidirectional actuation of ferrofluid using micropatterned planar coils assisted by bias magnetic fields

This paper reports a ferrofluid control method that enables both attraction and repelling of ferrofluid on micropatterned planar coils coupled with permanent magnets. A combinational use of a controlled magnetic field and a bias field is shown to provide lateral forces that attract/repel the ferrofluid to/from the coil depending on the direction of the current passed through the coil. Active mirror devices whose mirrors are switched by ferrofluids are developed as a proof-of-concept of the actuation method toward the application to imaging devices and optical switches. The planar devices lithographically fabricated to have arrays of mirror-coil cells are used to demonstrate activation/deactivation of individual cells enabled by the bidirectional radial motion of the ferrofluid layer with ∼100 μm thickness. The static and dynamic behaviors of the ferrofluid in the devices are characterized through an image processing approach. Multiple mirror cells are selectively and simultaneously operated to show enhanced ferrofluid control uniquely available with the two modes of the actuation as well as to demonstrate pattern generation with the arrays.     

Effect of dispersed phase viscosity on maximum droplet generation frequency in microchannel emulsification using asymmetric straight-through channels

Uniformly sized droplets of soybean oil, MCT (medium-chain fatty acid triglyceride) oil and n-tetradecane with a Sauter mean diameter of d _3,2 = 26–35 μm and a distribution span of 0.21–0.25 have been produced at high throughputs using a 24 × 24 mm silicon microchannel plate consisting of 23,348 asymmetric channels fabricated by photolithography and deep reactive ion etching. Each channel consisted of a 10-μm diameter straight-through micro-hole with a length of 70 μm and a 50 × 10 μm micro-slot with a depth of 30 μm at the outlet of each channel. The maximum dispersed phase flux for monodisperse emulsion generation increased with decreasing dispersed phase viscosity and ranged from over 120 L m⁻² h⁻¹ for soybean oil to 2,700 L m⁻² h⁻¹ for n-tetradecane. The droplet generation frequency showed significant channel to channel variations and increased with decreasing viscosity of the dispersed phase. For n-tetradecane, the maximum mean droplet generation frequency was 250 Hz per single active channel, corresponding to the overall throughput in the device of 3.2 million droplets per second. The proportion of active channels at high throughputs approached 100% for soybean oil and MCT oil, and 50% for n-tetradecane. The agreement between the experimental and CFD (Computational Fluid Dynamics) results was excellent for soybean oil and the poorest for n-tetradecane.     

Optical actuation of a bistable MEMS

This paper presents an optical actuation scheme for MEMS devices based on the well-established fact that light possesses momentum, and hence, imparts a force equal to 2 W/c when reflected by a surface. Here, W is the total power of the reflected light, and c is the speed of light. Radiation pressure, as it is known, is nearly insignificant for most macroscale applications, but it can be quite significant for MEMS devices. In addition, light actuation offers a new paradigm. First, intersecting light beams do not interfere, in contrast to electrical conductors, which short when they come into contact. Second, light can operate in high temperature and high radiation environments far outside the capability of solid state electronic components. This actuation method is demonstrated, both in air and in vacuum, by switching the state of a bistable MEMS device. The associated heat transfer model is also presented.     

Passive mixing in a three-dimensional serpentine microchannel

A three-dimensional serpentine microchannel design with a “C shaped” repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a “square-wave” channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring     

Efficiency of shock wave compression in a microchannel

The efficiency of compression processes tends to decrease dramatically when the overall dimensions are scaled down to micron level. Then it can be assumed that the compression is more efficient if no moving parts are involved in the process. This is achieved by replacing, at least partially, turbo-compressor stages with a wave rotor. The work starts with estimating wave rotor efficiency at microscale by extrapolation. Then, an analytical model is introduced from which the theoretical efficiency of compression process in a microchannel is deduced. Knowing the inlet conditions and the pressure gain across the shock, the overall efficiency of the compression can be calculated. The model assumes constant friction along the walls and no heat exchange with the surroundings. The results suggest that an efficiency of 70–80% can be achieved in the channels of an ultra-micro wave rotor. It is shown that if the inlet temperature is high enough (about 1500 K), the efficiency is even higher, up to 90%.     

Micro-droplet formation in non-Newtonian fluid in a microchannel

Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.