Eric Baird

Digitized Heat Transfer: A New Paradigm for Thermal Management of Compact Micro Systems

This paper presents theoretical and numerical results describing digitized heat transfer (DHT), a newly developing active thermal management technique for high-power density electronics and integrated micro systems. In describing DHT, we numerically investigate the mass, momentum, and energy equations governing the flow within a translating microdroplet. Our analysis shows the existence of a pair of recirculation zones inside the droplet. This internal circulation within discrete fluid slugs results in significantly increased overall heat transfer coefficients when compared to continuous Graetz-type flows. The internal circulation drives the cold fluid in the middle of the droplet to the vicinity of the walls and creates a higher local temperature difference between the wall and the fluid in contact with the wall, resulting in higher heat transfer rates. Nusselt numbers characterizing DHT flow are also shown to exhibit periodic fluctuations with a period equal to the characteristic time scale for droplet circulation. The overall effect of discretizing a flow on heat transfer capability is described and characterized in terms of a nondimensional circulation number defined by the ratio of characteristic thermal diffusion and fluid circulation time scales. DHT coolants, including liquid metals and alloys, are proposed, and their physical properties are shown to enable handling of significantly higher heat transfer rates than classical air- or water-cooled methods. The actuation method for DHT coolant transport is also outlined, and shown to provide the capability for active, on-demand suppression of transient hot spots. This overall analysis defines the key parameters for optimization of the DHT method and forms the basis of ongoing experimental work.

Digitized Heat Transfer Using Electrowetting on Dielectric

This paper proposes the use of electrowetting on dielectric (EWOD) as the driving force for digitized heat transfer (DHT), a novel approach to microscale thermal management in which system cooling is actively achieved via the manipulation of an array of discrete microdroplets. Galinstan, a nontoxic, readily available, inexpensive liquid alloy with 65 times less thermal resistance than water, is proposed as a viable DHT coolant. The nature of the EWOD driving force and the velocity of EWOD-actuated droplets are presented, along with an analysis demonstrating the advantages of DHT over some other methods of microscale heat control.

A Unified Velocity Model for Digital Microfluidics

This article presents a unified model for the velocity of discrete microdroplets. Simple algebraic expressions for steady-state droplet velocities are presented, as well as exact and approximate transient solutions. Specific results in terms of known experimental parameters are derived for the cases of electrowetting on dielectric (EWOD), dielectrophoresis (DEP), continuous electrowetting (CEW), and thermocapillary pumping (TCP). Model predictions are shown to agree with previously published theoretical and experimental results, giving fluid velocities for a broad range of applications in digitized microfluidics. A relative comparison of the models predictions for EWOD, CEW, DEP and TCP is also presented.

Digitized Heat Transfer for Thermal Management of Compact Microsystems

Active thermal management of compact microsystems by a periodic array of discrete liquid metal droplets is proposed and referred to as “digitized heat transfer.” This is in contrast to convective heat transfer by a continuous liquid flow. Two methods of droplet actuation, electrowetting on dielectric and continuous electrowetting, are described. Liquid metals or alloys support significantly higher heat transfer rates than other fluids, such as water or air. In addition, electrowetting is an efficient method of microscale fluid control, requiring low actuation voltages and very little power consumption. These concepts are used in this investigation to design an active management technique for high-power-density electronic and integrated micro systems. Preliminary calculations indicate that this technique could potentially offer a viable cooling strategy for achieving some of the most important objectives of electronic cooling, i.e., minimization of the maximum substrate temperature, reduction of the substrate temperature gradient and removing substrate hot spots. Numerical simulation of a droplet in a microchannel is also investigated. We propose a technique for dynamically calculating the slip velocity at the wall boundary including both the advancing and receding contact lines. The technique is based on the observed non-Newtonian behavior of a continuous liquid flow at high shear rates and its associated slip velocity (Thompson and Trioan 1997). While most of the wall boundary has negligible slip, significant slip at the advancing and receding contact lines are calculated from the data itself.Copyright

Digitized Heat Transfer: Thermal Management of Compact Micro Systems using Electrostatic Droplet Actuation

This paper presents theoretical and numerical results describing digitized heat transfer (DHT), a newly developing active management technique for high-power-density electronics and integrated micro systems. In DHT, thermal energy is transported by a discretized array of electrostatically activated microdroplets of liquid alloy or aqueous solution; internal circulation within each fluid slug results in significantly increased overall heat transfer coecients when compared to continuous Graetz-type flows. This paper identifies and describes a periodic fluctuation in Nusselt number resulting from this circulation eect. Proposed DHT coolants, especially liquid metals and alloys, support significantly higher heat transfer rates than classical air-cooled heat sinks, and can be digitally actuated and controlled with a high degree of both precision and programmability. As a consequence, DHT can be used both for steady state cooling of an entire integrated device and for precise, ecient, on-demand suppression of transient hot spots. A theoretical characterization of the governing parameters for DHT is presented, with a basic comparison of the heat transfer rates achieveable by liquid alloys and aqueous solutions. The eectiveness of DHT for managing both localized temperature spikes and steady state cooling is demonstrated. This analysis defines the key parameters for optimization of the DHT method and forms the basis of ongoing experimental work.

Electrowetting Droplet Manipulation for Application in Thermal Management of Compact Microsystems

Digitized heat transfer (DHT), a novel active management technique for high power density electronic and integrated micro systems in which heat is transported by a discrete array of electrostatically activated microdroplets, is proposed. Liquids, especially liquid metals or alloys, support significantly higher heat transfer rates than classical air-cooled heat sinks; in addition, discrete microdroplets are shown to be actuated and controlled with a high degree of precision and programmability. As a consequence, DHT is a viable new alternative for achieving the most important objectives of electronic cooling, i.e., minimization of the maximum substrate temperature, reduction of the substrate temperature gradient and removal of substrate hot spots. Three methods of microdroplet actuation, electrowetting on dielectric (EWOD), dielectrophoresis (DEP), and continuous electrowetting (CEW), are described, with simple results for steady state velocities in terms of known parameters. The use of EWOD to transport a droplet of commercially available liquid metal is reported. In addition, preliminary considerations on the heat transfer rates of such droplets are presented, with a simple analysis leading to a generalization of the continuous Nusselt number to a discretized flow.Copyright

Surface Tension Actuators Droplets in Microchannels

A unified model is presented for the velocity of discrete droplets in microchannels actuated by surface tension modulation. Specific results are derived for the cases of electrowetting on dielectric (EWOD), dielectrophoresis (DEP), continuous electrowetting (CEW), and thermocapillary pumping (TCP). This treatment differs from previously published works by presenting one unified analytic model which is then simply applied to the specific cases of EWOD, CEW, DEP and TCP. In addition, the roles of equiliubrium contact angle and contact angle hysteresis are unambiguously described for each method. The model is shown to agree with experimental and theoretical results presented previously, predicting fluid velocities for a broad range of applications in digitized microfluidics.Copyright

Electrostatic Droplet Actuation for Micro Scale Thermal Management

This paper presents a unified model for the velocity of microdroplets actuated by the application of a dc electric field. Simple algebraic expressions for steady state droplet velocities are presented, as well as exact and approximate transient solutions. Specific results in terms of known experimental parameters are derived for the cases of electrowetting on dielectric (EWOD), dielectrophoresis (DEP), and continuous electrowetting (CEW). The eect of contact angle hysteresis is included in the model in terms of advancing and receding contact angles, and velocity predictions are shown to agree with previously published theoretical and experimental results. The use of these actuation methods for active thermal management of compact microsystems via an array of discrete microdroplets, a process we refer to as ‘digitized heat transfer,’ is proposed. Preliminary calculations indicate that this technique will oer a viable cooling strategy for achieving the most important objectives of electronic cooling, i.e., minimization of the maximum substrate temperature, reduction of the substrate temperature gradient and removing substrate hot spots.