Shawn A. Putnam

All-optical beam deflection method for simultaneous thermal conductivity and thermo-optic coefficient ( dn/dT) measurements

This work describes an all-optical beam deflection method to simultaneously measure the thermal conductivity ( Λ) and thermo-optic coefficient ( dn/dT) of materials that are absorbing at λ= 10.6u2009μm and are transparent to semi-transparent at λ= 632.8u2009nm. The technique is based on the principle of measuring the beam deflection of a probe beam (632.8u2009nm) in the frequency-domain due to a spatially and temporally varying index gradient that is thermally induced by 50:50 split pump beam from a CO2 laser (10.6u2009μm). The technique and analysis methods are validated with measurements of 10 different optical materials having Λ and dn/dT properties ranging between 0.7u2009W/mu2009Ku2009 ≲Λ≲u200933.5u2009W/mu2009K and −12u2009×u200910−6u2009K−1u2009 ≲dn/dT≲u200914u2009×u200910−6u2009K−1, respectively. The described beam deflection technique is highly related to other well-established, all-optical materials characterization methods, namely, thermal lensing and photothermal deflection spectroscopy. Likewise, due to its all-optical, pump-probe nature, it is applicable to mate...

Simulating the evaporation of pinned water microdroplets with implementation of a surface concentration distribution

Numerical and experimental investigations of water microdroplet evaporation on polymer substrates are reported. The study is focused on validating numerical models with experimental data. The experimentally measured evaporation rates are compared to the predictions of a commonly reported model based on a solution of the Laplace equation, providing the local evaporation flux along the droplets liquid-vapor interface. The model consistently overpredicts the evaporation rate, which is presumable due to the models constant saturated vapor concentration along the droplets liquid-vapor interface. In result, a modified version of the model is implemented to account for variations in temperature along the droplets liquid-vapor interface. A vapor concentration distribution is then imposed using this temperature distribution, increasing the accuracy in predicted evaporation rate by ~ 7.7% and ~ 9.9% for heated polymer substrates at Ts = 50°C and 65°C, respectively.

Simultaneous reflectometry and interferometry for measuring thin-film thickness and curvature

A coupled reflectometer-interferometer apparatus is described for thin-film thickness and curvature characterization in the three-phase contact line region of evaporating fluids. Validation reflectometry studies are provided for Au, Ge, and Si substrates and thin-film coatings of SiO2 and hydrogel/Ti/SiO2. For interferometry, liquid/air and solid/air interferences are studied, where the solid/air samples consisted of glass/air/glass wedges, cylindrical lenses, and molded polydimethylsiloxane lenses. The liquid/air studies are based on steady-state evaporation experiments of water and isooctane on Si and SiO2/Ti/SiO2 wafers. The liquid thin-films facilitate characterization of both (i) the nano-scale thickness of the absorbed fluid layer and (ii) the macro-scale liquid meniscus thickness, curvature, and curvature gradient profiles. For our validation studies with commercial lenses, the apparatus is shown to measure thickness profiles within 4.1%-10.8% error.

Atmospheric Deposition of Modified Graphene Oxide on Silicon by Evaporation-Assisted Deposition

We present a deposition technique termed evaporation-assisted deposition (EAD). The technique is based on a coupled evaporation-to-condensation transfer process at atmospheric conditions, where graphene oxide (GO) is transferred to a Si wafer via the vapor flux between an evaporating droplet and the Si surface. The EAD process is monitored with visible and infrared cameras. GO deposits on Si are characterized by both Raman spectroscopy and X-ray photoelectron spectroscopy. We find that a scaled energy barrier for the condensate is required for EAD, which corresponds to specific solution–substrate properties that exhibit a minimized free energy barrier at the solid–liquid–vapor interface.

High‐Throughput, Protein‐Targeted Biomolecular Detection Using Frequency‐Domain Faraday Rotation Spectroscopy

A clinically relevant magneto-optical technique (fd-FRS, frequency-domain Faraday rotation spectroscopy) for characterizing proteins using antibody-functionalized magnetic nanoparticles (MNPs) is demonstrated. This technique distinguishes between the Faraday rotation of the solvent, iron oxide core, and functionalization layers of polyethylene glycol polymers (spacer) and model antibody-antigen complexes (anti-BSA/BSA, bovine serum albumin). A detection sensitivity of ≈10 pg mL-1 and broad detection range of 10 pg mL-1 ≲ cBSA ≲ 100 µg mL-1 are observed. Combining this technique with predictive analyte binding models quantifies (within an order of magnitude) the number of active binding sites on functionalized MNPs. Comparative enzyme-linked immunosorbent assay (ELISA) studies are conducted, reproducing the manufacturer advertised BSA ELISA detection limits from 1 ng mL-1 ≲ cBSA ≲ 500 ng mL-1 . In addition to the increased sensitivity, broader detection range, and similar specificity, fd-FRS can be conducted in less than ≈30 min, compared to ≈4 h with ELISA. Thus, fd-FRS is shown to be a sensitive optical technique with potential to become an efficient diagnostic in the chemical and biomolecular sciences.

Heat transfer coefficient measurements in the thermal boundary layer of microchannel heat sinks

This study describes the use of optical pump-probe diagnostics to characterize the heat transfer coefficient (HTC) in a developing thermal boundary layer in a microchannel. We use a differential form of the anisotropic time-domain thermoreflectance (TDTR) technique to measure the HTC as a function of fluid flow rate (or Reynolds number, Re). The testing environment/geometry consists of single-phase, degassed water flowing in a rectangular microchannel (hydraulic diameter Dh ≅ 480 μm) with local spot heating by the pump TDTR laser beam. Relative to the HTC measured with non-flowing (static) fluids, we find a 30% increase in the HTC for single-phase water flowing at Re ~ 1800.