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EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF MICRODROPLET EVAPORATION WITH A FORCED PINNED CONTACT LINE.
Title
EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF MICRODROPLET EVAPORATION WITH A FORCED PINNED CONTACT LINE.
Creator
Gleason, Kevin, Putnam, Shawn, University of Central Florida
Abstract / Description
Experimental and numerical investigations of water microdroplet evaporation on heated, laser patterned polymer substrates are reported. The study is focused on both (1) validating numerical models with experimental data, (2) identifying how changes in the contact line influences evaporative heat transfer and (3) determining methods of controlling contact line dynamics during evaporation. Droplets are formed using a bottom-up methodology, where a computer-controlled syringe pump supplies water...
Show moreExperimental and numerical investigations of water microdroplet evaporation on heated, laser patterned polymer substrates are reported. The study is focused on both (1) validating numerical models with experimental data, (2) identifying how changes in the contact line influences evaporative heat transfer and (3) determining methods of controlling contact line dynamics during evaporation. Droplets are formed using a bottom-up methodology, where a computer-controlled syringe pump supplies water to a ~200 um in diameter fluid channel within the heated substrate. This methodology facilitates precise control of the droplets growth rate, size, and inlet temperature. In addition to this microchannel supply line, the substrate surfaces are laser patterned with a moat-like trench around the fluid-channel outlet, adding additional control of the droplets contact line motion, area, and contact angle. In comparison to evaporation on non-patterned substrate surfaces, this method increases the contact line pinning time by ~60% of the droplets lifetime. The 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 liquid-vapor interface. A vapor concentration distribution is then imposed using this temperature distribution, increasing the accuracy of predicting the evaporation rate by ~7.7% and ~9.9% for heated polymer substrates at Ts = 50C and 65C, respectively.
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Date Issued
2014
Identifier
CFH0004566, ucf:45212
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFH0004566
Analysis of steady state micro-droplet evaporation to enhance heat dissipation from tiny surfaces.
Title
Analysis of steady state micro-droplet evaporation to enhance heat dissipation from tiny surfaces.
Creator
Voota, Harish, Putnam, Shawn, Kauffman, Jeffrey, Vasu Sumathi, Subith, University of Central Florida
Abstract / Description
Steady state droplet evaporation experiments are conducted to understand (1) Droplet contact line influence on evaporation rate and (2) Droplet contact angle correlation to evaporation rate. Experiments are performed on a polymer substrate with a moat like trench (laser patterned) to control droplet contact line dynamics. A bottom-up methodology is implemented for droplet formation on the patterned substrate. Droplet evaporation rates on substrate temperatures 22???T_Substrate?70? and contact...
Show moreSteady state droplet evaporation experiments are conducted to understand (1) Droplet contact line influence on evaporation rate and (2) Droplet contact angle correlation to evaporation rate. Experiments are performed on a polymer substrate with a moat like trench (laser patterned) to control droplet contact line dynamics. A bottom-up methodology is implemented for droplet formation on the patterned substrate. Droplet evaporation rates on substrate temperatures 22???T_Substrate?70? and contact angles 80(&)deg;???110(&)deg; are measured. For a pinned microdroplet (CCR), volumetric infuse rate influences droplet contact angle. Results illustrate droplet contact line impact on evaporation rate . Moreover, these results coincide with previously published results and affirm that evaporation rate efficiency reduces with contact line depinning. Additionally, from all the analyzed experimental cases, evaporation rate scales proportional to the microdroplet contact angle (i.e. ?_(LG )??). In conclusion, these experiments shed new light on steady state evaporation of a microdroplet and its corresponding observations. Vital research findings can be used to enhance heat dissipation from tiny surfaces.
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Date Issued
2015
Identifier
CFE0006235, ucf:51067
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006235
Three-phase contact line phenomena in droplets on solid and liquid surfaces: electrocapillary, pinning, wetting line velocity effect, and free liquid surface deformation
Title
Three-phase contact line phenomena in droplets on solid and liquid surfaces: electrocapillary, pinning, wetting line velocity effect, and free liquid surface deformation.
Creator
Shabani, Roxana, Cho, Hyoung, Kumar, Ranganathan, Kapat, Jayanta, Chow, Louis, Zhai, Lei, University of Central Florida
Abstract / Description
In this dissertation, physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wettingline) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL)phenomena, which encompass the wetting line energy (WLE), the wetting line velocity (WLV), and the contact anglehysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the air filmlubrication...
Show moreIn this dissertation, physical phenomena relevant to (i) an interface formed between two fluids and a solid phase (wettingline) and (ii) an interface between three fluids (triple contact line) were investigated. In the former case, the wetting line (WL)phenomena, which encompass the wetting line energy (WLE), the wetting line velocity (WLV), and the contact anglehysteresis, were studied using a micropump based on electrowetting on dielectric (EWOD). In the latter case, the air filmlubrication effect and the liquid free surface deformation were taken into account to explain the dual equilibrium states ofwater droplets on liquid free surfaces. A micropump based on droplet/meniscus pressure gradient generated by EWOD was designed and fabricated. By alteringthe contact angle between liquid and solid using an electric field a pressure gradient was induced and a small droplet waspumped into the channel. The flow rate in the channel was found to be constant in spite of the changes in the droplet'sradius. The WL phenomena were studied to unravel the physical concept behind the micropump constant flow rate. Theobservation and measurement reveal that the shrinking input droplet changes its shape in two modes in time sequence: (i)its contact angle decreases, while its wetting area remains constant, and (ii) its WL starts to move while its contact anglechanges. Contact angles were measured for the advancing and receding WLs at different velocities to capture a full pictureof contact angle behavior. The effects of the WLE on the static contact angle and the WLV on the dynamic contact angle inthe pump operation were investigated. Also the effect of EWOD voltage on the magnitude and uniformity of the micropumpflow rate was studied. Dynamic contact angles were used to accurately calculate the pressure gradient between the dropletand the meniscus, and estimate the flow rate. It was shown that neglecting either of these effects not only results in aconsiderable gap between the predicted and the measured flow rates but also in an unphysical instability in the flow rateanalysis. However, when the WLE and WLV effects were fully taken into account, an excellent agreement between thepredicted and the experimental flow rates was obtained.For the study of the TCL between three fluids, aqueous droplets were formed at oil-air interface and two stableconfigurations of (i) non-coalescent droplet and (ii) cap/bead droplet were observed. General solutions for energy and forceanalysis were obtained and were shown to be in good agreement with the experimental observations. Further the energybarrier obtained for transition from configuration (i) to (ii) was correlated to the droplet release height and the probability ofnon-coalescent droplet formation. Droplets formed on the solid surfaces and on the free surface of immiscible liquids have various applications indroplet-based microfluidic devices. This research provides an insight into their formation and manipulation.
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Date Issued
2013
Identifier
CFE0005253, ucf:50598
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005253