Niru Kumari

Electrical actuation of dielectric droplets

Electrical actuation of liquid droplets at the microscale offers promising applications in the fields of microfluidics and lab-on-a-chip devices. Much prior research has targeted the electrical actuation of electrically conducting liquid droplets; however, the actuation of dielectric droplets has remained relatively unexplored, despite the advantages associated with the use of a dielectric droplet. This paper presents modeling and experimental results on the electrical actuation of dielectric droplets between two flat plates. A first-order analytical model, based on the energy-minimization principle, is developed to estimate the electrical actuation force on a dielectric droplet as it moves between two flat plates. Two versions of this analytical model are benchmarked for their suitability and accuracy against a detailed numerical model. The actuation force prediction is then combined with available semi-analytical expressions for predicting the forces opposing droplet motion to develop a model that predicts transient droplet motion under electrical actuation. Electrical actuation of dielectric droplets is experimentally demonstrated by moving transformer oil droplets between two flat plates under the influence of an actuation voltage. Droplet velocities and their dependence on the plate spacing and the applied voltage are experimentally measured and showed reasonable agreement with predictions from the models developed.

Electrowetting-induced dewetting transitions on superhydrophobic surfaces.

We develop and demonstrate the use of electrowetting to achieve the dewetting (Wenzel-to-Cassie transition) of superhydrophobic surfaces. We effect this transition by means of an opposing flat plate and a three-electrode system; the liquid droplet is completely pulled out of its wetted Wenzel state upon the application of a suitable voltage. We also experimentally quantify the dissipative forces preventing the dewetting transition. The energy associated with these nonconservative forces is comparable to the interfacial energies.

Electrical Actuation of Electrically Conducting and Insulating Droplets using AC and DC Voltages

Electrical actuation of liquid droplets at the microscale offers promising applications in the fields of microfluidics and lab-on-chip devices. Much prior research has targeted the electrical actuation of electrically conducting liquid droplets using dc voltages (classical electrowetting). Electrical actuation of conducting droplets using ac voltages and the actuation of insulating droplets (using dc or ac voltages) has remained relatively unexplored. This paper utilizes an energy-minimization-based analytical framework to study the electrical actuation of a liquid droplet (electrically conducting or insulating) under ac actuation. It is shown that the electromechanical regimes of classical electrowetting, electrowetting under ac actuation and insulating droplet actuation can be extracted from the generic electromechanical actuation framework, depending on the electrical properties of the droplet, the underlying dielectric layer and the frequency of the actuation voltage. This paper also presents experiments which quantify the influence of the ac frequency and the electrical properties of the droplet on its velocity under electrical actuation. The velocities of droplets moving between two parallel plates under ac actuation are experimentally measured; these velocities are then related to the actuation force on the droplet which is predicted by the electromechanical model developed in this work. It is seen that the droplet velocities are strongly dependent on the frequency of the ac actuation voltage; the cut-off ac frequency, above which the droplet fails to actuate, is experimentally determined and related to the electrical conductivity of the liquid. This paper then analyzes and directly compares the various electromechanical regimes for the actuation of droplets in microfluidic applications.

Characterization of ultrahydrophobic hierarchical surfaces fabricated using a single-step fabrication methodology

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron-scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with the fabrication of two-tier roughness structures are eliminated. Three two-tier roughness surfaces with different roughness parameters are fabricated and tested. The surfaces are characterized in terms of the static contact angle and roll-off angle, and are compared with surfaces consisting of only single-tier microscale roughness. The evaporation characteristics of a sessile droplet on the hierarchical surfaces is also assessed relative to comparable single-roughness (SR) surfaces. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces exhibit very high contact angle and lower contact angle hysteresis compared to the SR surfaces and are more resistant to wetting. The energy loss during impact on the surfaces is quantified in terms of the coefficient of restitution for droplets bouncing off the surface.

Single-Step Fabrication and Characterization of Ultrahydrophobic Surfaces with Hierarchical Roughness

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with fabrication of two-tier roughness structures are eliminated. Experiments are conducted on two double-roughness surfaces with different surface roughness, achieved by varying the size of the microscale roughness features. The surfaces are characterized in terms of static contact angle and roll-off angle and compared with surfaces consisting of only single-tier microscale roughness. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces are more resistant to wetting than the single roughness surfaces and show higher coefficients of restitution for droplets bouncing off the surface. The droplet dynamics upon impingement are explored.Copyright

Analysis and Performance Comparison of Competing Desktop Cooling Technologies

The present work compares the performance of various competing thermal management technologies for the desktop sector. An air-cooled heat sink used for the Intel Pentium 4 Processor is used as the baseline for comparison. Heat sinks based on metal foams, microchannels (with single-phase liquid) and jet impingement (with air and single-phase liquid) are compared based on total heat sink system thermal resistance and heat dissipation capacity. The analysis is carried out under the constraints of a fixed heat sink volume available in a typical desktop, and a fixed ambient air temperature. The comparison of thermal resistances is made under the constraint of the same pumping power as in the baseline heat sink. The maximum heat dissipation possible using a particular heat sink technology is estimated and this can be used to select technologies to meet future thermal challenges as outlined in the International Technology Roadmap for Semiconductors (ITRS). The results show that microchannel and liquid jet impingement cooling provide the greatest heat removal rates under the given constraints. The maximum power dissipation for these cases is almost double that of the baseline air-cooled heat sink. Under the chosen constant value of the junction to heat sink resistance, only modest improvements in heat removal rate are obtained with the microchannel and jet impingement technologies even if the pumping power constraint is relaxed, and a specific pump curve is used instead. The junction to heat sink resistance is significantly higher than the heat sink to ambient resistance, and is the key determinant in the comparisons.Copyright