<p>Microscale
environments provide significant resolution and distortion challenges with
respect to measurement techniques; however, with improvements to existing
techniques, it is possible to gather relevant data to better understand the
thermal and fluidic mechanisms at such small scales in evaporating droplets.</p>
<p> </p>
<p>Infrared
thermography provides several unique challenges at small scales. A primary issue is that the low native
resolution of traditional infrared cameras significantly hamper the collection
of details of microscale features.
Furthermore, surfaces exhibiting vastly different emissivities, results
in inaccurate temperature measurements that can only be corrected with
irradiance-based emissivity maps of the surface; however, due to the resolution
limitations of infrared thermography, these emissivity maps can also display
significant errors. These issues are
overcome through the use of multi-frame super-resolution. The enhanced resolution allows for better
capture of microscale features, therefore, enhancing the emissivity map. A quantitative error analysis of the system
is conducted to quantify the feature size resolution improvement as well as the
smoothing effect of super-resolution reconstruction. Furthermore, a sensitivity analysis is
conducted to quantify the impact of registration uncertainty on the accuracy of
the reconstruction. Finally, the improved emissivity map from super-resolution
is demonstrated to show the increased accuracy over low-resolution mapping.</p>
<p> </p>
<p>When
applied to water droplets, particularly on nonwetting surfaces, infrared
thermography is confounded by the presence of nonuniform reflectivities due to
the spherical curvature of the liquid-air interface. Thus, when measuring the temperature along
the vertical axis of a water droplet, it is necessary to correct the
reflection. Using a controlled
background environment, in conjunction with the Fresnel equations, it is
possible to correct the reflective effects on the interface and calculate the
actual temperature profile. This allows
for a better understanding of the governing mechanisms that determine the
thermal transport within the droplet.
While thermal conduction is the primary transport mechanism along the
vertical axis of the droplet, it is determined that the temperature drop is
partially dampened by the convective transport from the ambient air to the
liquid interface. From this
understanding revealed by the measurements, the vapor-diffusion-based model for
evaporation was enhanced to better predict evaporation rates.</p>
<p> </p>
<p>Further
exploration into the mechanisms behind droplet evaporation on nonwetting
surfaces requires accurate knowledge of the internal flow behavior. In addition, the influence of the working
fluid can have a significant impact on the governing mechanisms driving the flow
and the magnitude of the flowrate. While
water droplet evaporation has been shown to be governed by buoyancy-driven
convection on nonwetting substrates, similar studies on organic liquid droplets
are lacking. Particle image velocimetry
is effective at generating a velocity flow field, but droplets introduce
distortion due to the refraction from the spherical interface of the
droplet. As such, velocity correction using
a ray-tracing approach was conducted to correct the velocity magnitudes and
direction. With the velocity
measurements, the flow was determined to be surface-tension-driven and showed speeds
that are an order of magnitude higher than those seen in buoyancy-driven flow
in water droplets. This resulted in the
discovery that advection plays a significant role in the transport within the
droplet. As such, the vapor-diffusion-governed
evaporation model was adjusted to show a dramatic improvement at predicting the
temperature gradient along the vertical axis of the droplet.</p>
<p> </p>
<p>Armed
with the knowledge of flow behavior inside droplets, it is expected that
droplets with aqueous solutions should exhibit buoyancy-driven convection. The final part of this work, therefore,
leverages this phenomenon to enhance mixing during reactions. Colorimetry is a technique that is widely
utilized to measure the concentration of a desired sample within some liquid;
the sample reacts with a reagent dye the color change is measured, usually
through absorbance measurements. In
particular, the Bradford assay is used to measure protein concentration by
reacting the protein to a Coomassie<sup>TM</sup> Brilliant Blue G-250. The absorbance of the dye increases, most
significantly at the 590 nm wavelength, allowing for precise quantitation
of the amount of protein in the solution.
A droplet-based reaction chamber with buoyancy-enhanced mixing has the
potential to speed up the measurement process by removing the need for a
separate pre-mixing step. Furthermore,
the reduced volume makes the process more efficient in terms of reactant
usage. Experimental results of premixed
solutions of protein sample and reagent dye show that the absorbance measurement
through a droplet tracks strongly with the protein concentration. When the protein sample and dye reagent are mixed
<i>in situ</i>, the complex interaction
between the reactants, the mixing, and the adsorption of protein onto the
substrate creates a unique temporal evolution in the measured absorbance of the
droplet. The characteristic peaks and valleys of this evolution track strongly
with concentration and provide the framework for measurement of concentration
in a droplet-based system.</p>
<p> </p>
<p>This thesis extends knowledge about droplet
thermal and fluidic behavior through enhanced measurement techniques. This knowledge is then leveraged in a novel
application to create a simple, buoyancy-driven colorimetric reaction setup. Overall, this study contributes to the field
of miniaturized, efficient reaction and measurement devices.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8044415 |
| Date | 14 May 2019 |
| Creators | Aditya Chandramohan (6635972) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Thermofluidic_Transport_in_Evaporating_Droplets_Measurement_and_Application/8044415 |
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