Modeling Electrowetting on Dielectric for Novel Droplet-Based Microactuation.

In this paper, EWOD (Electro-wetting on dielectric) system is used in reproductive medicine and has been expected to be a dynamic cell culture platform for assisted reproductive technology (ART). The EWOD device is anticipated to provide a stable culture environment and improve the growth environment of mouse embryos based on its advantages of uniform hormone mixing in the culture medium. As we predicted, the development rate of embryos cultured by dynamic process is better than embryos of static group. Although the development rate of the chip group is slightly worse than the control group, the biocompatibility of EWOD and the potential of EWOD chip for delivering biological cells and cells culture has been confirmed.

This paper describes the performance of a fluidic microrobot using Electrowetting on Dielectric (EWOD). A system to control the fluidic microrobot was designed, constructed and deployed in the NIST Mobile Microrobotics Challenge at ICRA 2011. The microrobots (0.1 M KCl and 550 μm diameter) demonstrated the ability to perform controlled maneuvers in 2-D while transporting hydrophilic objects. The EWOD system is composed of a DIP-mounted die produced via standard microfabrication techniques and containing the control electrodes / competition arena, and a transparent ITO cover slip for grounding. Key advantages of this platform include a scalable design for batch EWOD system fabrication, potential simultaneous control of multiple microrobots, and an easily portable, compact system design.

Optimum thickness of hydrophobic layer for operating voltage reduction in EWOD systems

This paper presents an Electrowetting-on-Dielectric (EWOD) device with optimized insulating layers operated by low actuation voltage. The device consists of an electrode array on a silicon substrate, covered by a dielectric layer and a hydrophobic layer. To characterize the performance of the device, simulations are performed for the dielectric layer of Sio2 and the hydrophobic layer of Sio2, Su-8 and Parylene C at different voltages. The volume finite difference approach of the Coventorware software was used to carry out the simulations. Two different molar of di-ionized water droplet were considered in the simulations. It was observed that the device having the Sio2 dielectric layer and the Parylene C hydrophobic layer moved the 1M KCL (potassium chloride) droplet at the actuation voltage of 25V.

We present a mixed finite element method for a model of the flow in a Hele-Shaw cell of 2-D fluid droplets surrounded by air driven by surface tension and actuated by an electric field. The application of interest regards a micro-fluidic device called ElectroWetting on Dielectric (EWOD). Our analysis first focuses on the time discrete (continuous in space) problem and is presented in a mixed variational framework, which incorporates curvature as a natural boundary condition. The model includes a viscous damping term for interface motion, as well as contact line pinning (sticking of the interface) and is captured in our formulation by a variational inequality. The semi-discrete problem uses a semiimplicit time discretization of curvature. We prove the well-posedness of the semi-discrete problem and fully discrete problem when discretized with iso-parametric finite elements. We derive a priori error estimates for the space discretization. We also prove the convergence of an Uzawa algorithm for solving the semi-discrete EWOD system with inequality constraint. We conclude with a discussion about experimental orders of convergence.

The detachment of soil by flowing water on a hillslope is dependent upon the hydraulics of shallow overland flows. The role of microtopography on controlling the spatial patterns of these flows is poorly understood. This study aimed to improve this understanding by undertaking a series of controlled laboratory tests using an experimental plot, consisting of a soil box, a rainfall simulator and overland flow generator. A particular emphasis was placed on developing low-cost photogrammetric techniques to provide novel, high spatial resolution measurements of surface DEMs, 2D flow velocities and distributions of inundation levels and fluid drag. Overland flow experiments were run over surfaces with differing topographies to further examine the effect of discharge and inundation ratio on the spatial patterns of overland flow velocities, hydraulic resistance and fluid drag. Structure-from-Motion (SfM) was used to measure the microtopography of soils eroded by high intensity rainfall events. An accuracy test showed, for the first time, that SfM is capable of producing highly accurate measurements of microtopographic features at a scale similar to those found on rilled soil surfaces. The results also provided crucial information on how to optimize the acquisition of SFM-imagery in different lightning situations and using different cameras. Analysis of the DEMs of the eroded surfaces revealed the potential to estimate soil surface roughness and the distribution of elevations based on plot slope. A low-cost Particle Tracking Velocimetry system was developed to provide novel, detailed 2D surface vector fields, co-located with the acquired DEMs. The results showed that streamwise and lateral velocities have high spatial variability, revealing the flow was 2D. The distribution shape of streamwise velocities was dependent upon discharge and thus relative submergence; the higher submergences, the spatial variability in streamwise velocity was higher and the distributions were wider, flatter and more skewed. The correlation between these distributions and the surface elevations was not sufficiently high to suggest that overland flow velocity can be predicted well by surface elevations alone. On the other hand, local roughness was better correlated, revealing that local variations in surface elevations are more important in controlling spatial patterns of overland flow velocity. The hydraulic resistance, estimated by the Darcy-Weisbach friction factor, varied strongly with the Reynolds number when the flows were fully turbulent but not when they were close to being transitional, such as in shallow, interrill flows. Surface roughness and relative submergence were found to have a greater or equally strong control as Reynolds number on hydraulic resistance. Drag force varied considerably over eroded soil surfaces, with some areas experiencing forces an order magnitude higher than others. Areas of high drag forces correlated with the walls of rills and with areas in which flows converged from different rills. Thus, the distributions of drag force provide some explanation for why rills expand, deepen and migrate, as well as why erosion rates are so variable over hillslopes. The drag force distributions were well-approximated by a two-parameter Weibull distribution, whose parameters were heavily dependent upon surface roughness and hydraulic resistance. The strength of these relationships revealed the potential to predict the drag force distribution, and thus spatial patterns of hydraulic resistance and erosion, based on these simple parameters. Overall, the study has provided the first detailed study of the effect of microphotography on overland flow patterns, revealing new understanding of the statistical properties of eroded surfaces and the control of roughness on the distribution of 2-D velocities, drag force and hydraulic resistance. These results have important implications for the modelling of overland flow and resultant flow detachment. In doing so, the study has demonstrated the potential of low-cost photogrammetric techniques for providing new understanding of the physics of overland flows, and thus, in the future, driving the development of more physically-based erosion models.

Dielectric breakdown is a common problem in a digital microfluidic system, which limits its application in chemical or biomedical applications. We propose a new fabrication of an electrowetting-on-dielectric (EWOD) device using Si3N4 deposited by low-pressure chemical vapor deposition (LPCVD) as a dielectric layer. This material exhibits a greater relative permittivity, purity, uniformity, and biocompatibility than polymeric films. These properties also increase the breakdown voltage of a dielectric layer and increase the stability of an EWOD system when applied in biomedical research. Medium droplets with mouse embryos were manipulated in this manner. The electrical properties of the Si3N4 dielectric layer-breakdown voltage, refractive index, relative permittivity, and variation of contact angle with input voltage-were investigated and compared with a traditional Si3N4 dielectric layer deposited as a plasma-enhanced chemical vapor deposition to confirm the potential of LPCVD Si3N4 applied as the dielectric layer of an EWOD digital microfluidic system.

Experimental studies of the magnetic fluid grinding process have previously established that it is an abrasive grinding process. Sliding between the material being ground (for example, a ceramic ball being finished) and the grinding surface occurs because of viscous drag forces and torques acting on the ground material as it is driven through a magnetic liquid. Mechanical models of the process have qualitatively predicted trends in grinding rates resulting from changes in operating conditions, but have not been quantitatively accurate. This paper establishes that the reason is inaccurate modelling, in the previous work, of the fluid drag forces and torques. Improved modelling now gives accurate prediction of the motions in a grinding cell. Interactions between balls travelling in a train and gyroscopic effects are also reported on; they are of secondary importance in influencing slip magnitude at the grinding surface but do provide mechanisms for randomizing ball motions.

The propulsive force generated by the leading-edge suction of a flapping thin airfoil can be obtained using unsteady aerodynamic notions based on the potential flow theory. Because the potential theory fails to predict the viscous forces, for flapping airfoils, in general there exists a net propulsive force generation. For low Reynolds numbers and for oscillations at low reduced frequencies, however, the viscous forces overcome the propulsive forces to give a net force against the flight direction. In the case of higher Reynolds numbers, flappings with large amplitudes, and high frequencies, the leading-edge suction force generated by the airfoil overcomes the viscous forces. In this study, the critical Reynolds number, the amplitude, and the reduced frequency values that yield the net propulsive force for an oscillating airfoil are predicted with unsteady viscous-inviscid interaction. The airfoil motions are modeled as 1) a thin rigid plate in vertical oscillation, that is, a heaving-plunging motion; 2) a flexibly cambered airfoil whose camber is changing periodically; and 3) the heaving-plunging motion of a flexibly cambered airfoil. The leading-edge suction force for all cases is predicted by means of the well-known Blasius theorem, and the time dependent surface velocity distribution of the airfoil is determined by unsteady aerodynamic considerations. This surface velocity distribution is used as the edge velocity of the unsteady boundary layer to predict the viscous effects. The coefficient of the propulsive force predicted with the present method agrees well with the values given in the literature up to the effective angle of attack at which the dynamic stall takes place.

Digital microfluidics (DMF) driven by electrowetting-on-dielectric (EWOD) has recently been attracting great attention as an effective liquid-handling platform for on-chip cooling. It enables rapid transportation of coolant liquid sandwiched between two parallel plates and drop-wise thermal rejection from a target heating source without additional mechanical components such as pumps, microchannels, and capillary wicks. However, a typical sandwiched configuration in DMF devices only allows sensible heat transfer, which seriously limits heat rejection capability, particularly for high-heat-flux thermal dissipation. In this paper, we present a single-sided digital microfluidic (SDMF) device that enables not only effective liquid handling on a single-sided surface, but also two-phase heat transfer to enhance thermal rejection performance. Several droplet manipulation functions required for two-phase cooling were demonstrated, including continuous droplet injection, rapid transportation as fast as 7.5 cm/s, and immobilization on the target hot spot where heat flux is locally concentrated. Using the SDMF platform, we experimentally demonstrated high-heat-flux cooling on the hydrophilic-coated hot spot. Coolant droplets were continuously transported to the target hot spot which was mitigated below 40 K of the superheat. The effective heat transfer coefficient was stably maintained even at a high heat flux regime over ~130 W/cm2, which will allow us to develop a reliable thermal management module. Our SDMF technology offers an effective on-chip cooling approach, particularly for high-heat-flux thermal management based on two-phase heat transfer.

Maglevs are typically accelerated using electromagnetic propulsion and levitation. High-temperature superconducting (HTS) magnets along with electrodynamic suspension (EDS) and linear synchronous motors are one of the best options for Hyperloop. However, the strong magnetic fields generated by HTS magnets on the pods inevitably interact with the magnetic and conductive structures in the vacuum tubes, along with the tube itself, while the pods move through the tubes. This interaction is observed as a drag force on the pods, significantly reducing the propulsion efficiency. This study comprehensively analyzes the electromagnetic drag force (EDF) generated by HTS magnets on pods, which accounts for most of the drag forces faced by Hyperloop. Theoretical analysis and 3D FEA simulations are performed to analyze the propulsion forces with HTS magnets and all the drag forces on the pods. The EDF generated by AISI 1010 steel rebars in concrete guideways is even greater than the designed propulsion forces of 40 kN. Consequently, high-manganese (Hi-Mn) steel and insulated steel rebars are adopted and analyzed using 3D FEA simulations. The EDFs generated by the AISI 1010 steel and Hi-Mn steel vacuum tubes are determined by varying the distance between the HTS magnets and tubes at 50 and 1200 km/h, respectively; a minimum distance of 0.75 m is determined by a drag force below 8 kN within their operating velocities. Lastly, the total EDFs of the AISI 1010 steel and Hi-Mn steel tubes with EDS rails are obtained through the optimal design of rebars and tubes. The simulation results show that the total EDFs can be significantly reduced to below 10 kN (approximately 25% of the designed propulsion force after the levitation of pods).

This study investigates the movement behavior of particles of dissimilar morphology in the powder bed in Laser Powder Bed Fusion. Gas atomized (GA) and water atomized (WA) low alloy steel powders were employed to study their motion around the laser scan path. Particle velocities, entrainment distances and denudation zones were measured for both powders using high-speed imaging. The entrainment of GA powder particles in front of the laser beam towards the process area was initiated 1.6 mm from the edge of the melt pool, whereas the distance was 0.6–0.8 mm for the WA powder. The differences in observed behavior were related to the variations in particle shape of the two types of powder. The processing of WA powder resulted in a 16% narrower denudation zone (for a low volumetric energy density) compared to GA powder. However, the denudation width difference decreased with increasing volumetric energy density, most likely due to a steeper pressure gradient in the process area which diminishes the impact of powder shape. X-ray computed microtomography was utilized to estimate the drag force acting on the powder particles of various morphologies. The results showed that the radial drag force exerted on GA powder was 64% greater than when using WA powder. Moreover, if the WA powder particles were of elongated shape the drag force decreased by almost an order of magnitude, demonstrating the importance of the particle's morphology in the process dynamics.

Many industries require contactless transport of delicate or clean products such as silicon wafers, flat foodstuffs and freshly painted objects. In this study, a contactless air film conveyor for flat objects is introduced. The object is supported by a thin film formed between the object and the conveyor surface and transported by viscous traction which is generated by controlled airflow underneath the object. Experiments are conducted to investigate the film pressure distribution and viscous force. The results show that the film pressure is symmetrically distributed along the direction perpendicular to the airflow in the actuating cells and non-symmetrically distributed along the airflow direction. The viscous force decreases as the increase of gap thickness. A simplified model is established to help understand and characterize the viscous force. The calculated results show a good agreement with the experimental data. It is revealed that the viscous force is the resultant of an actuating force in the pocket and side areas and a drag force from airflow across the dam area. Simulation and experiments are conducted using a PID controller for one dimension position control and position tracking. The results verify the effectiveness of the theoretical modeling and control method.

The reliability of the cargo door actuation system is closely related to the safety of civil aviation cargo aircraft. In order to ensure the safety of cargo aircraft, it is necessary to consider whether the engineering can be successfully started under the worst temperature and the worst opening conditions and to conduct a high-precision simulation. To improve the simulation accuracy, the finite element method and the field-circuit coupling method are analysed and applied. The electromagnetic field equation of the circuit is calculated using the Maxwell platform, and the control circuit equation calculated by the Simplorer platform is simulated at the same time. The load side of the motor is analysed accurately and the simulation authenticity is greatly improved. To successfully start the motor in a harsh environment, the influence of high temperature on the motor is analysed to compensate for the reduction of output torque caused by high temperature. The motor is designed repeatedly to achieve the task goal. The final simulation results meet the requirements and can overcome the harsh temperature environment and the harsh open-angle environment, which provides ideas about the development and simulation of the door electrical actuation system.

Considerable efforts have been devoted to self-propelled microrockets during the past decades for its diverse potential applications, ranging from targeted drug delivery to environmental remediation. Particularly attention has been given to the chemical-driven rockets which are analogous to actuators and can convert chemical energy into mechanical energy to achieve a certain propulsion velocity or force. Since the drag force on the surface of microrockets dominates the resistance in the microfluidic systems, it is of interest to decrease the drag force for efficient propulsion. A new microrocket with lower drag force is proposed with a hydrophobic molecular layer, 1-Dodecanethiol layer, on the outer polyaniline surface of the rockets by using self-assembly technology. The contact angle and the velocity of microrockets are determined before and after surface modification to quantify the drag force reduction. The findings of the research indicate that reducing drag force is a promising way for comprehensive application.