The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. Temperature-sensitive (Rhodamine B) and temperature-insensitive (Sulforhodamine-101) fluorescent dyes are used in tandem to determine fluid temperature with high accuracy and low noise using a pulsed Nd:YAG laser as an illumination source. While the fluorescence intensity of the temperature-sensitive dye is proportional to temperature, it is also biased by variations in the illuminating intensity. Therefore, a second temperature-insensitive dye is required to compensate for such biases. Calibration of the two-color LIF system using the RhB-SR101 dye combination in ethanol and water yields temperature sensitivities of −1.5% K −1 and −2.7% K −1 , respectively, with volumetric illumination from an Nd:YAG laser. The feasibility of this methodology for conducting temperature measurements is explored by measuring a steady-state temperature gradient maintained across a microfluidic channel array by two large hot and cold reservoirs. These measurements reveal that the mean steady-state temperatures in the microchannels are within ±0.4 • C and ±0.3 • C of the predicted temperatures with ethanol and water as the solvents, respectively, with a spatial resolution of 22.2 × 22.2 μm. The experimental uncertainties in the measurements using the RhB-SR101 dye combination are ±0.48-0.59 • C and ±0.41-0.49 • C for ethanol and water, respectively.

Partially polymerized membranes display a striking mechanical transition at low temperature known as the wrinkling transition. Fluorescence and scanning electron microscopy as well as profile measurements using an atomic force microscope revealed the existence of three degrees of wrinkling depending on the degree of the membrane polymerization. At low polymerization the membrane undergoes a cascade of wrinkling to form a folded phase with a characteristic exponent eta equal to 3, at intermediate polymerization, the membrane is in an intermediate-wrinkled phase (similar to the crumpling of an elastic sheet) with eta approximately 2.5, while at high polymerization the membrane undergoes an abrupt "compaction" to the wrinkled-rough phase with eta approximately 2.

The present effort documents the relationship between dominant subgrid-scale energy transfer events and coherent motions within the log layer of wall turbulence. Instantaneous velocity fields in the streamwise-wall-normal plane of a zero-pressure-gradient turbulent boundary layer acquired by particle-image velocimetry at Re ϵ u * ␦ / = 2350 are spatially filtered to generate an ensemble of resolved-scale velocity fields in the spirit of large-eddy simulation. The relationship between subgrid-scale dissipation and embedded coherent structures is then studied using instantaneous realizations and conditional averaging techniques. This analysis reveals that strong forward-and backward-scatter events occur spatially coincident to individual hairpin vortices and their larger-scale organization into vortex packets. In particular, large-scale regions of forward scatter are observed along the inclined interface of the packets, coincident with strong ejections induced by the individual vortices. The most intense forward-scatter events are found to occur when these ejections are opposed by sweep motions. Strong backward scatter of energy is observed at the trailing edge of the vortex packets and weaker backscatter is also noted locally around the individual heads of the hairpin structures. The collective observations presented herein demonstrate that hairpin vortices and their organization into larger-scale packets are important contributors to interscale energy transfer in the log layer of wall turbulence.

The spatial signatures of retrograde spanwise vortices in wall turbulence are assessed from particle-image velocimetry measurements in the streamwise-wall-normal plane of a zero-pressure-gradient turbulent boundary layer at Re τ ≡ u * δ/ν = 2350. The present results suggest that a proportion of retrograde spanwise vortices have a welldefined spatial relationship with neighbouring prograde vortices. Two-point crosscorrelations and conditionally averaged velocity fields given a retrograde vortex reveal that such structures are typically oriented either upstream of and below or downstream of and above a prograde core. While these pairings are consistent with the typical-eddy patterns reported by Falco and co-workers, we offer an alternative interpretation for a proportion of these retrograde/prograde pairs. In particular, the arrangement of a retrograde spanwise vortex upstream of and below a prograde core is also consistent with the spatial signature revealed if an omega-shaped hairpin structure were sliced through its shoulder region by a fixed streamwise-wall-normal measurement plane.

Two-and three-dimensional microvascular networks embedded within a polymer fin were fabricated via direct write assembly to demonstrate cooling potential of vascular polymer structures. Thin fin cooling experiments were carried out utilizing water and polyalphaolefin (PAO) oil-based coolant as the working fluids. The surface temperature of the fin was monitored using an infrared camera and flow distribution within the network was evaluated by microscopic particle image velocimetry. The effective heat transfer coefficient was increased 53-fold at low Reynolds number for water cooling in both 2D and 3D geometries. However, 3D architectures offer more uniform flow distribution and the ability to efficiently adapt to blockages and reroute flow within the network. Microvascular materials are excellent candidates for compact, efficient cooling platforms for a variety of applications and 3D architectures offer unique performance enhancements.