Ridha Ben-Mrad

An Empirically Validated Analytical Model of Droplet Dynamics in Electrowetting on Dielectric Devices

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.

A Piezoactuated Droplet-Dispensing Microfluidic Chip

A microfluidic dispensing device that is capable of generating droplets with volumes varying between 1 nL and 50 pL at an ejection frequency of up to 6 kHz is presented. In this device, a piezoactuator pushes onto an elastic membrane via piston tips; the mechanical bending of the membrane generates a pressure pulse pushing droplets out. An analytical model was developed solving bending characteristics of a plate-actuated fluidic dispensing system and used to calculate the displaced volume. The model was extended to perform stress analysis to find the optimum piston tip radius by minimizing design stresses. The optimum piston tip radius was found to be 67% of the chamber radius. The actuation force estimated using the analytical model was then used as input to a finite element model of the dispenser. A detailed numerical analysis was then performed to model the fluid flow and droplet ejection process and to find critical geometric and operating parameters. Results from both models were used together to find the best design parameters. The device contains three layers, a silicon layer sandwiched between two polydimethylsiloxane (PDMS) polymer layers. Silicon dry etching, together with PDMS soft lithography, was used to fabricate the chip. PDMS oxygen plasma bonding is used to bond the layers. Prototypes developed were successfully tested to dispense same-sized droplets repeatedly without unwanted droplets. The design allows easy expansion and simultaneous dispensing of fluids.

A Shear-Mode Energy Harvesting Device Based on Torsional Stresses

The shear-mode of a piezoceramic material offers improved energy harvesting potential as compared to other modes. This paper presents a concept of using torsional stresses induced by nonrotational vibrations in an energy harvesting device to produce electrical power. A finite-element model of this concept is presented to illustrate the principle and a prototype is demonstrated to validate the design. This prototype harvester is found to be capable of producing a maximum power of 0.57 mW at its resonant frequency of 620 Hz with a base acceleration of 1 g in amplitude. When compared to conventional cantilever harvester designs, this torsion-based harvester is found to suffer from fewer electrical power losses and has a greater potential in producing high power outputs.

Discrete-time compensation algorithm for hysteresis in piezoceramic actuators

A discrete-time Preisach model that captures hysteresis in a piezoceramic actuator is developed. The model is implemented using a numerical technique that is based on first order reversal functions and is presented in a recursive form that is amenable for real-time implementation. The first order reversal functions are experimentally obtained using a piezoceramic actuator in a stacked form. The development model shows good agreement with actual measured data. A hysteresis compensation scheme based on the developed discrete-time Preisach model is also developed and used in order to obtain a linear voltage-to-displacement relationship.

Mechanical Filtration of Particles in Electrowetting on Dielectric Devices

A passive mechanical method for the filtration of particles in electrowetting on dielectric (EWOD) devices is presented. Analytical and experimental results show that droplets actuated by EWOD cannot pass physical obstructions unaided at the scales considered here. However, it was possible to pull droplets past the same obstructions using a second droplet. The two droplets approach the obstruction from opposite sides and merge within the pore of the obstruction. The interface on the enabling side of the amalgamated droplet is then actuated to pull fluid through the obstruction. This technique was successful for pore sizes between half and two orders of magnitude below the confined droplet height. This wide range of viable pore sizes will allow for the filtration of particles by size in EWOD devices. It can also be used to filter large particles traditionally used in microfluidic immunoassays or allow for the use of smaller particles to increase sensitivity. Success at pore sizes as small as 2 μm also suggests that filtration of animal cells in EWOD devices is possible. The proposed process is performed without the use of surfactants, which may make it more attractive for applications using biological material.

Electrowetting on Dielectric (EWOD)-Based Thermo-Responsive Microvalve for Interfacing Droplet Flow With Continuous Flow

This paper reports on a hybrid electrowetting-thermal passive microvalve for a flow control mechanism to interface droplet flow with continuous flow. When heated, a thermo-responsive polymer, in the form of a dilute aqueous droplet, forms a gel, and upon cooling, it liquefies. The gel blocks and its subsequent liquefaction unblocks the pressurized flow. Electrowetting on dielectric (EWOD) actuation was used to control and position a polymer droplet at a desired valving location. The valve interfaced a pressurized flow to a planar EWOD-based droplet flow in a single device. It was fabricated using a standard planar microfluidic glass platform. It has a three-layer architecture bonded by oxygen plasma bonding. The valve demonstrated zero leakage and no cross-mixing across the interface.

Development of a Piezoelectric Fuel Injector

A piezoelectric fuel injector that directly controls the injection process using a piezoelectric actuator is presented. In the design pursued, energizing the actuator initiates the displacement of the injection needle and allows the fuel spray to form. The direct-acting concept is implemented by developing a motion inverter that reverses the direction of the input force and displacement and amplifies the output displacement. A fuel injector prototype was built, consisting of an actuator module that housed the preloaded piezoelectric actuator, a motion inverter module, and a dispensing module consisting of the injection nozzle and needle. Experiments with input voltage signals corresponding to those used in diesel engines verified the performance of the fuel injector prototype. The prototype was successfully tested for 250-, 400-, 600-, and 800-

A Drop-on-Demand-Based Electrostatically Actuated Microdispenser

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A 3-DOF MEMS Electrostatic Piston-Tube Actuator

holding times and only showed a response delay due to hysteresis, which can be reduced using closed-loop control with the integration of the electronic control unit (ECU). Since the prototype takes advantage of direct control of the injection needle, it allows for more precise control of the timing of the injections. The design also has the potential to control the quantity of injected fuel more precisely than currently available commercial injectors.

A MEMS micromirror based head-up display system

This paper presents a noncontact drop-on-demand three-layer microdroplet generator based on electrostatic actuation. The dispenser is actuated via a deformable membrane that isolates the electrical field from the working fluid. The dispenser controlled droplet formation, frequency, size, and velocity within the ranges tested. Prototypes were fabricated using three-step deep reactive-ion etching and polydimethylsiloxane (PDMS) plasma activated bonding. Experiments verified stable droplet dispensing with a variance in subsequent droplet volume of less than 15% between droplets. The frequency of stable generation was 20 Hz, and the average volume of dispensed droplet was 1 nL. The dispenser operating range and the nondestructive actuation make it suitable for biological applications.