Fabrication of metallic micromolds by laser and electro-discharge micromachining

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper. The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet. The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate. The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing. To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS). The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra. The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4. Experimental results are discussed and suggestions for improvement are presented.     

Streaming currents in microfluidics with integrated polarizable electrodes

A surface in contact with an aqueous solution is electrically characterized by the zeta potential. One way of determining indirectly the zeta potential of a surface is by measuring the streaming currents generated by a Poiseuille flow through a capillary channel with charged walls. We report measurements of streaming current in individual rectangular glass/PDMS microchannels with integrated miniaturized electrodes. Experiments performed using solutions with different salt concentrations and different electrode materials showed that the measured electrical current depends on the electrode material and in general differs from the real value of the streaming current. To determine the streaming current from the experimental data, an equivalent circuit model is proposed. The extracted value of the streaming current is proportional to the flow rate of electrolyte and the calculated glass/PDMS zeta potential scales linearly with the logarithm of the salt concentration. This work offers a thorough analysis of the effects that come into play during streaming current measurements and, in particular, it describes potential sources of error that can affect the streaming current measurements and suggestions on how to correct the measured values.     

Design and fabrication of microfluidic mixer from carbonyl iron�CPDMS composite membrane

This paper introduces a carbonyl iron–PDMS (CI–PDMS) composite magnetic elastomer in which carbonyl iron (CI) particles are uniformly distributed in a PDMS matrix. The CI particles and the PDMS were mixed at different weight ratios and tested to determine the influence of CI concentration. The magnetic and mechanical properties of the magnetic elastomers were characterized, respectively, by vibrating-sample magnetometer and by tensile testing using a mechanical analyzer. The elastomer was found to exhibit high magnetization and good mechanical flexibility. The morphology and deformation of the CI–PDMS membrane also were observed. A magnetically actuated microfluidic mixer (that is, a micromixer) integrated with CI–PDMS elastomer membranes was successfully designed and fabricated. The high efficiency and quality of the mixing makes possible the impressive potential applications of this unique CI–PDMS material in microfluidic systems.     

Fabrication and characterization of a passive silicon-based direct methanol fuel cell

This paper reports on the fabrication and characterization of a passive silicon microfabricated direct methanol fuel cell (μDMFC). The main characteristics of the device are its capability to work without complex pumping systems, only by capillary pressure, and the fact that its performance is not affected by the device orientation. A simple fabrication process based in deep reactive ion etching (DRIE), allows obtaining a reliable and low-cost final device. The device consists of two silicon microfabricated plates mounted together with a commercial membrane electrode assembly (MEA). The impact of current collector design on microfuel cell performance is explored and current–voltage (I–V) and current–power (I–P) curves of the device at different methanol concentration and orientation are presented. Optimal performance was obtained for methanol concentrations between 3 and 5 M, achieving a maximum power density of 12 mW/cm². The results obtained in this work demonstrate the feasibility of the device and give a guideline for design and conditions optimization.     

Single-walled carbon nanotube network/poly composite thin film for flow sensor

A fabrication method about single-walled carbon nanotube (SWCNT) network and polydimethylsiloxane (PDMS) based composite thin film is reported, which can be used as flow sensor cell. This composite thin film is immersed in deionized water and salt solution with different flow rate tests. The morphology of SWCNTs on the surface of the composite thin film is characterized by scanning electron microscopy, revealing the SWCNTs are coated by PDMS chains. The induced voltage generates along the direction of the flowing liquid and depends significantly on the ionic concentration and flow velocity. Since the SWCNTs are fixed into PDMS matrix, the I–V curves of the composite thin film before and after several flow velocity measurements are exactly coincident, and the repeating flow-induced voltage experiment shows the composite thin film has a reliable electric characteristic and wide potential of device application.     

Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars

Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with high-speed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the non-uniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 μm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.     

Chaotic mixing using source�Csink microfluidic flows in a PDMS chip

We present an active fixed-volume mixer based on the creation of multiple source–sink microfluidic flows in a polydimethylsiloxane (PDMS) chip without the need of external or internal pumps. To do so, four different pressure-controlled actuation chambers are arranged on top of the 5 μl volume of the mixing chamber. After the mixing volume is sealed/fixed by microfluidic valves made using ‘microplumbing technology’, a virtual source–sink pair is created by pressurizing one of the membranes and, at the same time, releasing the pressure of a neighboring one. The pressurized air deforms the thin membrane between the mixing and control chambers and creates microfluidic flows from the squeezed region (source) to the released region (sink) where the PDMS membrane is turned into the initial state. Several schemes of operation of virtual source–sink pairs are studied. In the optimized protocol, mixing is realized in just a sub-second time interval, thanks to the implementation of chaotic advection.     

Experimental characterization of the temperature dependence of zeta potential and its effect on electroosmotic flow velocity in microchannels

This study attempts to characterize the influence of temperature on zeta potential for a number of commonly used buffers in both poly(dimethylsiloxane) (PDMS):glass and PDMS:PDMS microchannels. The study is motivated by the apparent inability of the Smoluchowski equation for electroosmotic flow (EOF) velocity, U = [ε ₀ ε _r ζ/μ]E _z , to accurately predict EOF velocities at elevated temperatures. Error can result if zeta potential (ζ) is taken to be constant, even if permittivity (ε) and viscosity (μ) are treated as temperature-dependent variables. In some cases, velocity may be underestimated by more than 30%. In this study, the time-interval current-monitoring method was used to measure zeta potential. A hotplate maintained precise channel temperatures and applied electric field strengths were selected so that Joule heating was negligible. Results show that in some solutions (e.g., KCl, TBE), the zeta potential can exhibit a strong dependence on temperature, changing by as much as 50% over a span of 60°C. This influence was found to increase with solution concentration. Other buffers (e.g., TE, Na₂CO₃/NaHCO₃) were stable over all measured temperatures.     

Mixing and flow regulating by induced-charge electrokinetic flow in a microchannel with a pair of conducting triangle hurdles

The induced-charge electrokinetic flow (ICEKF) in a rectangular micorchannel with a pair of conducting triangle hurdles embedded in the middle is investigated in this paper. A correction method is suggested to numerically estimate the induced zeta potential on the conducting surface. Two-dimensional pressure-linked Navier-–Stokes equation is used to model the flow field in the channel. The numerical results show flow circulations generated from the induced non-uniform zeta potential distribution along the conducting hurdle surfaces. It is demonstrated numerically that the local flow circulations provide effective means to enhance the flow mixing between different solutions; by adjusting the electric field applied through the microchannel with a non-symmetric triangle hurdle pair, an electrokinetic flow regulating effect can be obtained and this effect depends on the dimensions of the conducting converging–diverging section. The mixing and flow regulating using ICEKF described in this paper can be used in various microfluidics and lab-on-a-chip (LOC) applications.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

A simple method for interpolating meshes of arbitrary topology by Catmull–Clark surfaces

Interpolating an arbitrary topology mesh by a smooth surface plays important role in geometric modeling and computer graphics. In this paper we present an efficient new algorithm for constructing Catmull–Clark surface that interpolates a given mesh. The control mesh of the interpolating surface is obtained by one Catmull–Clark subdivision of the given mesh with modified geometric rule. Two methods—push-back operation based method and normal-based method—are presented for the new geometric rule. The interpolation method has the following features: (1) Efficiency: we obtain a generalized cubic B-spline surface to interpolate any given mesh in a robust and simple manner. (2) Simplicity: we use only simple geometric rule to construct control mesh for the interpolating subdivision surface. (3) Locality: the perturbation of a given vertex only influences the surface shape near this vertex. (4) Freedom: for each edge and face, there is one degree of freedom to adjust the shape of the limit surface. These features make interpolation using Catmull–Clark surfaces very simple and thus make the method itself suitable for interactive free-form shape design.     

Pressure-sensitive channel chip for visualization measurement of micro gas flows

We have developed a new pressure sensing tool named pressure-sensitive channel chip (PSCC) by combining the pressure-sensitive paint (PSP) technique with the poly(dimethylsiloxane) (PDMS) micro-molding technique. The PSP technique based on the oxygen quenching of luminescence is a potential diagnostic tool for pressure measurement of micro gas flows. However, the application of PSP to micro scale measurement is very difficult, because the thickness and the surface roughness of conventional PSPs cannot be neglected compared with the characteristic length of micro channels, and the spatial resolution is not enough for micro scale measurements due to the aggregations of luminophore. PSCC is fabricated with PDMS containing a pressure-sensitive luminophore; thus PSCC is a micro channel which itself works as a pressure “distribution” sensor. A micro converging-diverging nozzle with the throat width of 120 μm was demonstrated. The pressure distribution on the nozzle surface was successfully obtained by PSCC without the shortcomings of conventional PSPs.     

Noncontact impedance sensing

This article discusses noncontact impedance sensing used to measure the mass, viscosity, and stiffness of an environment. The developed sensor is composed of a laser displacement sensor and an air nozzle for the force supply. The key point of the design is that we use a common axis for both force probing and distance sensor so that we can obtain the exact displacement of the environment. We also discuss the appropriateness of the estimated parameters from the viewpoint of sensitivity of response. Experimental results are shown to confirm the effectiveness of the proposed sensor. We also tested the developed sensor on pears to confirm the possibility of measuring their surface impedance. This work was presented in part at the 10th International Symposium on Artificial Life and Robotics, Oita, Japan, February 4–6, 2005     

Transient analysis of sliding contact of layered elastic/plastic solids with rough surfaces

 Based on the boundary element method (BEM) and a variational principle, a numerical model is developed to analyze the time – transient sliding contact of two layered elastic/plastic solids. Two cases are considered: one is the loading/sliding/unloading of a rough surface on a smooth surface, and the other is of two rough surfaces. Contact statistics, contact pressure profile and stress distribution are predicted at each time step with updated surface roughness. The results are used to study the effect of surface roughness, physical properties of the layer and the substrate, and lubricant film thickness on friction, stiction, and wear. Discussion on the integration of this contact model into advanced tribological models, e.g., wear model, is also presented. Received: 28 June 2002/Accepted: 23 October 2002 Currently at: Seagate Technology, Pittsburgh, PA Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Polydimethylsiloxane, a photocurable rubberelastic polymer used as spring material in micromechanical sensors

 Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable photocurable silicone rubber which has a unique flexibility (G≈250 kPa) at room temperature. Further properties of PDMS are a low elasticity change versus temperature (1.1 kPa/°C), no elasticity change versus frequency and a high compressibility. PDMS is an interesting polymer to be used as spring material in micromechanical sensors such as accelerometers. The spring constant of the PDMS structures was theoretically calculated and measurements were done on accelerometers with PDMS springs to validate the theory. The measured and calculated spring constants showed a good correspondence, so the measurement results showed that the PDMS structures can successfully be used as mechanical springs. Received: 29 October 1996 / Accepted: 13 November 1996     

Electroosmotic flow at a liquid–air interface

This paper presents the first experimental evidence on electroosmotic flow at a liquid–air interface. A PDMS microchannel with an opening to air was created to allow for the formation of a liquid–air interface. Polystyrene particles were used to visualize the liquid motion and the experiments found that the particle velocity at the liquid–air interface was significantly slower than the particle velocity in the bulk. This result agrees with a mathematical model that considers the effects of electrical surface charges at the liquid–air interface in electroosmotic flow.     

Diffusioosmosis of electrolyte solutions in fibrous porous media

An analytical study is presented for the diffusioosmotic flow of an electrolyte solution in the fibrous medium constructed by an ordered array of parallel charged circular cylinders at the steady state. The prescribed electrolyte concentration gradient is constant but can be oriented arbitrarily with respect to the axes of the cylinders. The electric double layer surrounding each cylinder may have an arbitrary thickness relative to the radius of the cylinder. A unit cell model which allows for the overlap of the double layers of adjacent cylinders is employed to account for the effect of fibers on each other. The electrostatic potential distribution in the fluid phase of a cell is obtained by solving the linearized Poisson–Boltzmann equation, which applies to the case of low surface potential of the cylinders. The macroscopic electric field induced by the imposed electrolyte concentration gradient through the fluid phase in a cell is determined as a function of the radial position. A closed-form formula for the fluid velocity profile of the electrolyte solution due to the combination of electroosmotic and chemiosmotic contributions as a function of the porosity of the array of cylinders correct to the second order of their surface charge density or zeta potential is derived as the solution of a modified Navier–Stokes equation. The diffusioosmotic velocity can have more than one reversal in direction over a small range of the zeta potential. For a given electrolyte concentration gradient in a cell, the fluid flow rate does not necessarily increase with an increase in the electrokinetic radius of the cylinder, which is the cylinder radius divided by the Debye screening length. The effect of the radial distribution of the induced axial electric field in the double layer on the diffusioosmotic flow is found to be of dominant significance in most practical situations.     

Transition from Cassie�CBaxter to Wenzel States on microline-formed PDMS surfaces induced by evaporation or pressing of water droplets

In this work, we directly observed the evolution of air/water interfaces suspended between polydimethylsiloxane (PDMS) microlines when water droplets reduced their sizes due to evaporation. The inclined angles of the microline sidewalls were slightly larger than 90°. Two important phenomena were observed regarding the transition from Cassie–Baxter to Wenzel States. First, when a water droplet gradually shrank, an air/water interface between two neighboring microlines increased its deflection but decreased its angle with the vertical direction. In the meanwhile, the two edges of this interface were still at the top corners of the two microlines. Second, once water passed the top corners of these two microlines, it kept moving down and filled the gap. Based on these two phenomena, the equilibrium of a triple line and the uniformity of pressure inside a small water droplet, critical values of droplet sizes and Laplace pressure were derived to predict when the transition would occur on microlines. The derived theoretical relationships indicate that air/water interfaces may be stationary on both top corners and sidewalls of microlines if the inclined angles of the microline sidewalls are less than 90°. Otherwise, the interfaces can only be stationary at the top corners of the microlines. The predicted values of droplet sizes for the case that the inclined angles of these sidewalls are larger than 90° were validated by experimental results on three arrays of PDMS microlines. In addition, we also directly observed the evolution of air/water interfaces on PDMS microlines when a water droplet was slowly pressed using a glass slide. The critical values of the droplet sizes derived in the case of evaporation applied to this pressing case as well, and had a good match with experimental results on the three arrays of PDMS microlines. In addition to the cases of evaporation and pressing, the theoretical relationships derived in this work may also apply to other cases, in which Laplace pressure is gradually increased inside a liquid droplet and half sizes of the droplet are less than the capillary length of the liquid. Finally, based on developed transition criteria, a set of criteria were also proposed to design microlines for reducing the critical droplet size that triggers the transitions from Cassie–Baxter to Wenzel States.     

Control–structural design optimization for vibration of piezoelectric intelligent truss structures

Based on the design sensitivity analysis for structural dynamics in time domain, an integrated control–structural design optimization method is proposed to the vibration control of piezoelectric intelligent truss structure. In this investigation, the objective function and constraint functions include not only the conventional design indexes of structure but also the vibration control indexes and the feedback control variables. The structural design variables are optimized simultaneously with the vibration control system. The sensitivity relations for the control–structure optimization model are derived by using a new method, and the sequential linear programming algorithm is used to solve this kind of optimization problem. The numerical examples given in the paper demonstrate the effectiveness of methods and the program.     

Optimal tracking control of a flexible hub–beam system with time delay

In this paper, a new technique of time-delay compensation is proposed for active control of a flexible hub–beam system. The first-order approximation coupling (FOAC) model proposed recently for dynamics of hub–beam systems is used to verify the applicability of this technique. The FOAC model is first linearized to obtain a linearized equation. The linearized equation with time delay is then transformed into a standard form with no time delay by a particular integral transformation. The time-delay controller is designed based on this standard equation using the classical optimal tracking control theory. Since the controller is a function of modal coordinates, a modal filter is presented to estimate the modal coordinates from physical sensor measurements. The effectiveness of the proposed technique for time delay is demonstrated by numerical simulations. Simulation results indicate that a very small time delay may result in instability of the control system if it is not compensated in control design. The proposed time-delay controller is effective in controlling the system even when the maximum time delay for stability without time-delay compensation is greatly exceeded. Moreover, for the system without time delay, the proposed time-delay controller may possibly obtain much better control effectiveness than the controller without time delay.