S. Ravi Annapragada

Dynamics of droplet motion under electrowetting actuation

The static shape of droplets under electrowetting actuation is well understood. The steady-state shape of the droplet is obtained on the basis of the balance of surface tension and electrowetting forces, and the change in the apparent contact angle is well characterized by the Young-Lippmann equation. However, the transient droplet shape behavior when a voltage is suddenly applied across a droplet has received less attention. Additional dynamic frictional forces are at play during this transient process. We present a model to predict this transient behavior of the droplet shape under electrowetting actuation. The droplet shape is modeled using the volume of fluid method. The electrowetting and dynamic frictional forces are included as an effective dynamic contact angle through a force balance at the contact line. The model is used to predict the transient behavior of water droplets on smooth hydrophobic surfaces under electrowetting actuation. The predictions of the transient behavior of droplet shape and contact radius are in excellent agreement with our experimental measurements. The internal fluid motion is explained, and the droplet motion is shown to initiate from the contact line. An approximate mathematical model is also developed to understand the physics of the droplet motion and to describe the overall droplet motion and the contact line velocities.

Analysis of Gap Formation in the Casting of Energetic Materials

The problem of undesirable separation of the cast material from the mold in the casting of energetic materials is investigated. Comprehensive models are developed to simulate the heat and mass transfer processes during melt casting of energetic materials, as well as the resulting thermal stresses induced. The thermal and stress models are dynamically coupled. Predictions from the validated numerical model show excellent agreement with experimental measurements. The size and location of the separation are also predicted by the present model. Means to control and suppress separation are explored, and it is demonstrated that the separation can be controlled through proper choice of cooling conditions.

Determination of Electrical Contact Resistivity in Thermoelectric Modules (TEMs) From Module-Level Measurements

An experimental apparatus was developed to characterize the performance of a thermoelectric module (TEM) and heat sink assembly when the TEM was operated in refrigeration mode. A numerical model was developed to simulate the experiments. Bulk and interfacial Ohmic heating, the Peltier effect, Thomson effect and temperature-dependent bulk material properties, i.e., Seebeck coefficient and electrical conductivity were considered. A novel, self-consistent characterization methodology was developed to obtain the electrical contact resistivity at the interconnects in a TEM from the numerical simulations and the experiments. The electrical contact resistivity of the module tested was determined to be approximately 1.0 × 10-9 Ωm . The predictions are consistent with electrical contact resistivity obtained based on the performance specifications (ΔTmax) of the TEM.

Analysis and Suppression of Base Separation in the Casting of a Cylindrical Ingot

This paper presents a comprehensive numerical investigation of the influence of cooling conditions on base separation, void formation, and thermally induced stresses during the solidification of a high Prandtl number energetic melt in a cylindrical enclosure. Numerical models have been developed to simulate the heat and mass transfer processes in melt casting as well as analyze the base separation and thermal stresses induced during solidification. Two models are dynamically coupled, and the numerical predictions are validated against experiments. Based on the numerical analysis, modified cooling conditions are suggested that are shown to reduce base separation.

Solidification Heat Transfer and Base Separation Analysis in the Casting of an Energetic Material in a Projectile

This paper investigates the problem of base separation in the casting of energetic materials in a projectile. Special challenges that arise in casting high Prandtl number energetic materials in projectiles of complex geometries are addressed. A comprehensive numerical model is developed by integrating finite volume and finite element methods to analyze the thermal and flow fields as well as the residual stresses. The predictions, which are confirmed by experimental measurements, suggest that sustenance of a linear temperature profile along the projectile axis can eliminate base separation, and also reduce residual stresses in the final casting.Copyright

Droplet Shapes on Superhydrophobic Surfaces Under Electrowetting Actuation

Droplet behavior on structured surfaces has recently generated a lot of interest due to its application to self-cleaning surfaces and in microfluidic devices. In this paper, the droplet shape and the droplet state on superhydrophobic surfaces are predicted using the Volume of Fluid (VOF) approach. Various structured surfaces are considered and the apparent contact angles are extracted from the predicted droplet shapes. Droplet dynamics under electrowetting are also modeled, including contact line friction. The model is validated against in-house experiments and experiments from the literature. The droplet state, droplet shape and apparent contact angles match well with the experimental measurements. The Cassie and Wenzel states on structured surfaces are also accurately predicted. Further, the electrowetting-induced transition from the Cassie to the Wenzel state and the reversal to the Cassie state is predicted for two different superhydrophobic surfaces. The transient wetting process, intermediate energy states and droplet shapes during electrowetting are simulated. The effective contact line friction coefficient on pillared surfaces is predicted to be 0.14 Ns/m2 , consistent with published values.Copyright

Forces Acting on Sessile Droplet on Inclined Surfaces

Although many analytical, experimental and numerical studies have focused on droplet motion, the mechanics of the droplet while still in its static state, and just before motion starts, are not well understood. A study of static droplets would shed light on the threshold voltage (or critical inclination) for initiating electrically (or gravitationally) induced droplet motion. Before the droplet starts to move, the droplet shape changes such that the forces acting at the triple contact line balance the actuation forces. These contact line forces are governed by the contact angles along the contact line. The contact angle varies from an advancing angle at the leading edge to a receding angle at the trailing edge of the droplet. The present study seeks to understand and predict these forces at the triple contact line. The droplet shape, as well as the advancing and receding contact angles, is experimentally measured as a function of droplet size under the action of a gravitational force at different inclination angles. The advancing and receding contact angles are correlated with static contact angle and Bond number. A Volume of Fluid - Continuous Surface Force model with varying contact angles along the triple contact line is developed to predict the same. The model is first verified against a two-dimensional analytical solution. It is then used to simulate the shape of a sessile droplet on an incline at various angles of inclination and to determine the critical angle of inclination as a function of droplet size. Good agreement is found between experimental measurements and predictions. The contact line profile and contact area are also predicted. The contact area predictions based on a spherical-cap assumption are also compared against the numerical predictions.Copyright

Permeability and Thermal Transport in Compressed Open-Celled Foams

A computational methodology is proposed to describe the fluid transport in compressed open-celled metallic foams. Various unit-cell foam geometries are numerically deformed under uniaxial loads using a finite element method. An algorithm is developed and implemented to deform the fluid domain mesh inside the unit-cell foam based on the deformed solid unit-cell geometry. Direct simulations of the fluid transport in these deformed meshes are then performed over a range of Reynolds numbers used in practical applications. The model is validated against available experimental results and correlations. A corrected model is proposed for the permeability of compressed foams as a function of strain for flows transverse to the direction of compression. The thermal conductivity of fluid-saturated foams is also computed. Compression of foams increases the conductivity transverse to the direction of compression and decreases the conductivity parallel to it.Copyright