Michael J. Schertzer

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.

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.

Transient Interface Shape and Deposition Profile Left by Desiccation of Colloidal Droplets on Multiple Surfaces

Control of deposition patterns left by desiccated colloidal droplets is valuable in applications ranging from medical diagnostics to inkjet printing. This investigation presents an experimental method to monitor the transient interface shape of a colloidal droplet during desiccation and to quantify the deposition pattern left by the colloidal material optically. Transient image profiles and particle deposition patterns are examined for droplets containing fluorescent particles that were desiccated on glass and on the photoresist SU-8 3005. Contact line pinning was more prevalent on glass, where the contact diameter remained approximately constant throughout the process and the contact angle decreased with time. On SU-8, the contact diameter was initially constant, but decreased after the contact angle was reduced. The initial contact diameter on glass was similar to the diameter of the deposition pattern. The diameter of the deposition pattern on SU-8 was approximately half of the initial contact diameter. The deposition on SU-8 was also observed to be more uniform than that left on glass. These results suggest that selection of an appropriate substrate is an important consideration for colloidal deposition. The method presented will be used to in future investigations to characterize the effectiveness of coffee stain suppression through the application of external electric fields.Copyright

Analytical Models to Determine the Electric Field Characteristics of a Multi-Electrode Impedimetric Immunosensor in a Digital Microfluidic Device

The level of integration of digital microfluidics is continually increasing to include the system path from fluid manipulation and transport, on to reagent preparation, and finally reaction detection. Digital microfluidics therefore has the capability to encompass all steps of common biochemical protocols. Reported here is a set of analytical models for the design of a coplanar interdigitated multi-electrode array to be used as an impedimetric immunosensor in a digital microfluidic device for on-chip chemical reaction detection. The models are based on conformal mapping techniques, and are compared to results obtained from finite element analysis to discuss limitations of the model. The analytical models are feasible and inexpensive surrogates for numerical simulation methods.Copyright

Performance of an electrostatic actuated micromirror in a vacuum and non-vacuum packaging

Experimental results of micromirrors sealed in vacuum and non-vacuum environments are presented. The micromirror is 1.0 mm in diameter and is supported by four electrostatic repulsive actuators. The micromirror exhibits linear out-of-plane motion and rotational motion in two axes. The measured data shows that translational motion is 71 μm and 76 μm at 200 V for vacuumed and non-vacuumed micromirrors, respectively. The maximum optical rotational displacements for the vacuum and non-vacuum packaged micromirrors are 1.0° and 1.1°, respectively at 160 V. Packaging the micromirror in a vacuum decreases the squeeze film damping in the system. The settling time for the vacuum packaged micromirrors is 75 ms with an average overshoot of 116%. The settling times for the non-vacuumed micromirror are 2.75 ms with 5% overshoot for downward motion and 3.32 ms with 48% overshoot for upward motion. The estimated resonant frequency of the vacuum packaged micromirror is 2900 Hz, whereas the resonant frequency for the non-vacuumed micromirror is 1400 Hz. The static and dynamic results for the micromirror in reduced pressures determined that effects approaching the breakdown voltage become evident, resulting in lower displacements, and squeeze film damping effects are mitigated, leading to more consistent performance characteristics such as settling time and percent overshoot.

Characterizing the surface quality and droplet interface shape for microarray plates.

The variation in the surface quality of microarray plates was examined by measuring the contact angles of 480 droplets on five microarray plates. It was found that the measured contact angle did not accurately predict the droplet shape for moderate Bond numbers (~0.5 ≤ N(B) ≤ 5). By defining an apparent contact angle using the ratio of the contact radius to the height, the variance in the predicted interface shape decreased by greater than a factor of 3 for both local and globally averaged characteristics. The error in the predicted droplet height was also reduced by 3 orders of magnitude.

Characterization of Novel High Performance Lubricants

This work will provide a method to characterize a variety of novel high performance lubricants. In particular, surface tension and evolution of the contact angle on a variety of surfaces over one hour were recorded. Contact angles were measured using a rame-hart Goniometer. Surface tension is measured with the same device using the pendant drop method. Fluids studied here include: trihexyltetradecylphosphonium bis(trifluoromethyl-sulfonyl) amide ([THTDP][NTf2]), trihexyltetradecyl-phosphonium decanoate ([THTDP][Deca]), 1-ethyl-3-methyl imidazolium trifluorosulfonyl imide ([EMI][NTf2]), and 1-ethyl-3-methyl imidazolium trifluoromethanesulfonate ([EMI][FMS]). Contact angles were measured on the following surfaces: AISI 52100 stainless steel disks polished to 0.01–0.05 μm, Kapton, SU-8, Teflon, and glass slides. The resultant change in roughness on 52100 steel disks was measured to provide insight into the corrosive properties of each liquid.Copyright

Electrowetting on Dielectric (EWOD) Assisted Droplet Desiccation

Lab-on-a-chip (LOAC) devices are emerging technologies that aim to perform all of the laboratory functions of traditional diagnostic tests on single microchips. Microarrays are one promising type of LOAC device that consist of an array of droplets for testing tens to thousands of samples simultaneously. Microarrays are commonly used in gene sequencing, pathogen detection, determining microbial resistances, and conducting enzyme-linked immunosorbent assays (ELISAs). As droplets in these arrays dry, the majority of material within the droplet is deposited around the periphery. This phenomenon is referred to as the coffee stain effect. The non-uniform depositions left by this effect can result in variation of fluorescence intensity measurements in automated vision systems. A means of producing more uniform particle depositions for the microscopy analysis would allow for more accurate test results.One promising method for suppression of the coffee stain effect involves the use of electrowetting on dielectric (EWOD). EWOD devices apply an electrokinetic force at the three-phase contact line to manipulate the shape of a droplet interface. The Mugele group has already begun investigating EWOD’s effects on the coffee stain effect and found that an AC voltage applied to droplets on EWOD devices can suppress the coffee stain effect and produce smaller, more uniform droplet deposition patterns.This work presents (i) a method to characterize the deposition pattern left by a desiccated droplet as a function of radial position and (ii) a discussion of the microfabrication technique used to create devices to perform EWOD assisted desiccation for both AC and DC voltages.Copyright

Impedimetric Feedback in Particle Laden Digital Microfluidic Devices

Impedimetric measurement methods are a novel approach to the characterization of fluid in biological applications. Lab on a chip (LOAC) technologies could be combined with impedimetrics to benefit these applications. LOAC devices are currently being developed to pursue the miniaturization of larger scale processes. Current research shows great flexibility in using LOAC devices to reproduce biological processes such as those used in medical diagnostic applications. With a smaller form factor, testing that generally requires off-site lab usage can be deployed at the point-of-care. LOAC devices also have the potential to lower operating costs by reducing reagent volumes, labor costs, and cycle times.Digital microfluidic devices (DMF) are one promising LOAC platform. These devices manipulate discrete droplets of fluid using electric fields. As such, DMF devices can create, move, merge, and mix droplets while eliminating mechanical components like channels, pumps, and valves. Manipulation of discrete volumes over a planar array of electrodes allows for the possibility of highly flexible, reconfigurable devices.Addressable positions on a DMF device have conductive planes above and below the droplets which form a parallel plate capacitor. Using this principle, the electrical properties of the system can be measured in the same circuit that is used for droplet manipulation, removing the need for additional sensing components. This research tests the hypothesis that the impedance of a particle laden droplet in a DMF device can be modelled using an equivalent circuit model for particles that span more than half the gap height. The fundamental understanding gained increases sensitivity in impedimetric measurements, and can also be used for DMF applications in medical diagnostics, cell manipulation and observation, and condition based maintenance. This research presents an analytical model based on an equivalent circuit of a particle laden droplet. The proposed model predicts that droplet impedance is a function of device geometry, particle size, particle concentration, and the electrical properties of the particles and the surrounding medium.Copyright

Characterization of bead-based reactions and mechanical supernatant dilution in digital microfluidic devices

Discrete flow microfluidic devices have been identified as a technology that can be used to efficiently deliver health care services by reducing the cycle times and reagent consumption of biological protocols and medical diagnostic procedures. These devices also have the potential to reduce overhead costs by performing these applications at the point of care. Digital microfluidic devices are one promising discrete flow microfluidic platform that can individually create, manipulate, and mix droplets through the application of asymmetric electric fields. The work presented here suggests that (1) monitoring electrical properties of droplets during specific chemical reactions and (2) supernatant dilution via mechanical filtration can be integrated into digital microfluidic immunoassay devices. Measurement of electrical properties during the bead based chemical reaction between p-nitrophenyl phosphate and particles with bound antibodies conjugated to alkaline phosphatase resulted in measureable difference in capacitance and resistance when compared to the control particles. A single filtration cycle using the mechanical supernatant dilution method demonstrated here reduced the fluorescence intensity in particle laden droplet by approximately 80%.