Farzad Houshmand

Superhydrophobic Graphene Foams

The static and dynamic wetting properties of a 3D graphene foam network are reported. The foam is synthesized using template-directed chemical vapor deposition and contains pores several hundred micrometers in dimension while the walls of the foam comprise few-layer graphene sheets that are coated with Teflon. Water contact angle measurements reveal that the foam is superhydrophobic with an advancing contact angle of ∼163 degrees while the receding contact angle is ∼143 degrees. The extremely water repellent nature of the foam is also confirmed when impacting water droplets are able to completely rebound from the surface. Such superhydrophobic graphene foams show potential in a variety of applications ranging from anti-sticking and self-cleaning to anti-corrosion and low-friction coatings.

Graphene Drape Minimizes the Pinning and Hysteresis of Water Drops on Nanotextured Rough Surfaces

Previous studies of the interaction of water with graphene-coated surfaces have been limited to flat (smooth) surfaces. Here we created a rough surface by nanopatterning and then draped the surface with a single-layer graphene sheet. We found that the ultrasheer graphene drape prevents the penetration of water into the textured surface thereby drastically reducing the contact angle hysteresis (which is a measure of frictional energy dissipation) and preventing the liquid contact line from getting pinned to the substrate. This has important technological implications since the main obstacle to the motion of liquid drops on rough surfaces is contact angle hysteresis and contact line pinning. Graphene drapes could therefore enable enhanced droplet mobility which is required in a wide range of applications in micro and nanofluidics. Compared to polymer coatings that could fill the cavities between the nano/micropores or significantly alter the roughness profile of the substrate, graphene provides the thinnest (i.e., most sheer) and most conformal drape that is imaginable. Despite its extreme thinness, the graphene drape is mechanically robust, chemically stable, and offers high flexibility and resilience which can enable it to reliably drape arbitrarily complex surface topologies. Graphene drapes may therefore provide a hitherto unavailable ability to tailor the dynamic wettability of surfaces for a variety of applications.

Piranha Pin-Fins (PPF): Voracious boiling heat transfer by vapor venting from microchannels - system calibration and single-phase fluid dynamics

A novel approach to embedded electronics cooling with a multi-phase microfluidic heat sink termed the Piranha Pin-Fin (PPF) is presented. Several first-generation PPF devices, as well as plain-channel and solid pin-fin heat sinks, have been fabricated and experimentally tested under single-phase adiabatic conditions. Details of the PPF device geometry and microfabrication process are provided. Plots showing pressure drop and friction factor are also provided. Numerical fluid dynamics modeling has been performed in parallel to the experiments. Modeling data presented includes fractional flow through the pins, predicted pressure losses, fluid streamlines and velocity gradients under several operating conditions. Additionally, micro-particle image velocimetry (μPIV) measurements have been performed. The velocity fields are used to provide further insight into the fluid mechanics within the heat sink as well as to validate the models. Velocity field measurements are included for various operating conditions.

Active control of flow boiling oscillation amplitude and frequency using a transverse jet in crossflow

We demonstrate a technique to mitigate thermal oscillations in microchannel flow boiling and suppress the characteristic frequency associated with these oscillations. The method employs a transverse jet in crossflow that is fabricated along with the primary microchannel in a double-sided vinyl tape, using laser machining. Liquid at ambient temperature is injected into a flow boiling region at different momentum flux ratios to control the local temperature. A maximum reduction of 82% in temperature fluctuations was demonstrated and the dominant frequency of oscillations was completely suppressed within a particular range of momentum flux ratios. The observed phenomena are attributed to the replenishment of liquid into dryout regions, thereby preventing the large temperature rise and subsequent drop caused by dryout and rewetting, respectively.

Transient wall temperature measurements of two-phase slug flow in a microchannel

A High frequency wall temperature measurement approach at the microscale is demonstrated. This experimental approach was implemented to study the transient effect of bubbles in a slug flow regime. Air stream was injected into liquid water flow in a 210 μm deep and 1.5 mm wide horizontal microchannel to form a slug flow regime (with bubble frequency of ~280 Hz) and the associated wall temperature variations were recorded using the embedded resistance temperature detectors (RTDs) inside the channel-on the heater area. Synchronized images of the two-phase flow were simultaneously recorded by a high speed camera, and the recorded footage was used to interpret the observed trends. Three different regions with different heat transfer characteristics were identified for each cycle of bubble/liquid-slug passage.

Heat Transfer Enhancement in Micro-Domains Using Gas-Driven Liquid Film

A liquid film has been introduced upstream of a heater in a microchannel with gas flow, and the impact on the heat transfer performance has been investigated. The shear force exerted by the gas flow on the gas-liquid interface drives the film and drags it downstream, onto the heated area. Distilled water was injected through a 350 μm circular hole in a main stream of Nitrogen in a 220 μm deep and 1.5 mm wide rectangular microchannel to enhance the heat transfer from a 1 mm × 1 mm heater. Average heat transfer coefficient was studied for different gas and liquid flow rates and compared with single-phase flow. Significant improvement in heat transfer performance was observed while the pressure drop in the channel was not increased dramatically.Copyright