Engineering and Technology Abstract ID# : 573 ABSTRACT ...
Post-doctoral Research Associate Category: Engineering and Technology Abstract ID# : 573
Characterization of Transport Properties in Superhydrophobic Microchannels to Improve Lab-on-a-chip Applications GOAL
ABSTRACT Fluid flow over slippery surfaces is known to reduce friction thus decreasing the required pumping power in microchannels. Such slippery surfaces can be achieved from trapping gas within the rough walls, a state also known as the Cassie state. While this state is conventionally believed to decrease wall friction1 and increase convection2, less attention has been given to verify this belief by comparing the hydrodynamic and heat transfer effects between the Cassie state and partial/fully wetted state (Wenzel state). This poster presents our work to quantify the advective heat transfer efficiency and slip velocity of fluid flow over rough walls using twocolor fluorescence thermometry and micro-particle image velocimetry under different wetting criteria. The walls of the microchannels are corrugated with micro-ribs arranged transverse to the fluid flow. Based on our previous results3,4, we expect that the Cassie state will actually increase the microchannel friction and impede the heat removal rate compared to the Wenzel state.
INTRODUCTION
Tae Jin Kim* Carlos H. Hidrovo** *sites.google.com/a/utexas.edu/taejinkim/ **www.coe.neu.edu/research/hidrovo
HEAT TRANSFER CHARACTERIZATION
1. Validate how shear-free the boundary condition is under room temperature conditions 2. Study the heat removal efficiency of heated superhydrophobic microchannels
SLIP CHARACTERIZATION
Question to be answered: Does the bubble mattress enhance heat removal rate? Measurement method: Two-color fluorescence thermometry Overview: 1. Apply heat from the side walls of the superhydrophobic microchannel 2. Measure the corresponding advective heat transfer efficiency with the aid of temperature sensitive fluorescence dyes (a) Wetted state
• • • 1.
(b) De-wetted state
Question to be answered: How shear-free are the air pockets? Measurement method: Micro-particle image velocimetry (μ-PIV) Overview: Map out the velocity field in superhydrophobic microchannels under varying wetting modes 2. Quantify the corresponding slip length incident on the wall
(a) Wetted state
(b) De-wetted state
Major hurdle in developing Lab-on-a-chip devices 1. Large hydrodynamic resistance:
Portable Pump
0.3 (W)
2. Heating power:
Film Heater
1.0(W)
3. Limited power supply:
AAA Battery
1.05(Wh)
Example of two-color fluorescence thermometry under (a) Wenzel state and (b) Cassie state
(a)
Pressure tap
(b)
m(c pTb )
How can we reduce friction/enhance convection?
Qheater
Superhydrophobic Surfaces! Droplets resting on top of a superhydrophobic surface
Tb μ-PIV results of velocity field (black arrows) and streamlines (red lines) for penetration depth of (a) 155μm and (b) 5μm
(a)
(b) U y 0 bslip
Wenzel state
Microheater
du dy y 0
(a) Micrograph of superhydrophobic microchannel with integrated heaters and (b) graph of advective efficiency vs. Re
Slip length
CONCLUSION
Cassie-Baxter state
Conventional belief in microchannels with superhydrophobic surfaces 1. Shear free boundary condition effectively reduces friction 2. Slip flow enhances convective heat transfer
Period
• Friction over the bubble mattress may actually be substantial
References
• The Wenzel mode may induce larger friction reduction effects than the Cassie mode
1J.
P. Rothstein, Annual Review of Fluid Mechanics 42, 89-109 (2010). 2D. Maynes and J. Crockett, Journal of Heat Transfer, vol. 136, p. 011701, 2014. 3T. J. Kim and C. H. Hidrovo, Physics of fluids, vol. 24, pp. 112003-112003-18, 2012. 4T. J. Kim, R. Kanapuram, A. Chhabra, and C. H. Hidrovo, Transactions of the ASME-I-Journal of Fluids Engineering, vol. 134, p. 114501, 2012.
Graph of (a) sample of a velocity profile with slip flow near the wall and (b) the average slip length per period for varying penetration depth
• The Wenzel mode may be more efficient in removing heat than the smooth microchannel and the Cassie mode superhydrophobic microchannel
Characterization of Transport Properties in Superhydrophobic Microchannels to Improve Lab-on-a-chip Applications GOAL
ABSTRACT Fluid flow over slippery surfaces is known to reduce friction thus decreasing the required pumping power in microchannels. Such slippery surfaces can be achieved from trapping gas within the rough walls, a state also known as the Cassie state. While this state is conventionally believed to decrease wall friction1 and increase convection2, less attention has been given to verify this belief by comparing the hydrodynamic and heat transfer effects between the Cassie state and partial/fully wetted state (Wenzel state). This poster presents our work to quantify the advective heat transfer efficiency and slip velocity of fluid flow over rough walls using twocolor fluorescence thermometry and micro-particle image velocimetry under different wetting criteria. The walls of the microchannels are corrugated with micro-ribs arranged transverse to the fluid flow. Based on our previous results3,4, we expect that the Cassie state will actually increase the microchannel friction and impede the heat removal rate compared to the Wenzel state.
INTRODUCTION
Tae Jin Kim* Carlos H. Hidrovo** *sites.google.com/a/utexas.edu/taejinkim/ **www.coe.neu.edu/research/hidrovo
HEAT TRANSFER CHARACTERIZATION
1. Validate how shear-free the boundary condition is under room temperature conditions 2. Study the heat removal efficiency of heated superhydrophobic microchannels
SLIP CHARACTERIZATION
Question to be answered: Does the bubble mattress enhance heat removal rate? Measurement method: Two-color fluorescence thermometry Overview: 1. Apply heat from the side walls of the superhydrophobic microchannel 2. Measure the corresponding advective heat transfer efficiency with the aid of temperature sensitive fluorescence dyes (a) Wetted state
• • • 1.
(b) De-wetted state
Question to be answered: How shear-free are the air pockets? Measurement method: Micro-particle image velocimetry (μ-PIV) Overview: Map out the velocity field in superhydrophobic microchannels under varying wetting modes 2. Quantify the corresponding slip length incident on the wall
(a) Wetted state
(b) De-wetted state
Major hurdle in developing Lab-on-a-chip devices 1. Large hydrodynamic resistance:
Portable Pump
0.3 (W)
2. Heating power:
Film Heater
1.0(W)
3. Limited power supply:
AAA Battery
1.05(Wh)
Example of two-color fluorescence thermometry under (a) Wenzel state and (b) Cassie state
(a)
Pressure tap
(b)
m(c pTb )
How can we reduce friction/enhance convection?
Qheater
Superhydrophobic Surfaces! Droplets resting on top of a superhydrophobic surface
Tb μ-PIV results of velocity field (black arrows) and streamlines (red lines) for penetration depth of (a) 155μm and (b) 5μm
(a)
(b) U y 0 bslip
Wenzel state
Microheater
du dy y 0
(a) Micrograph of superhydrophobic microchannel with integrated heaters and (b) graph of advective efficiency vs. Re
Slip length
CONCLUSION
Cassie-Baxter state
Conventional belief in microchannels with superhydrophobic surfaces 1. Shear free boundary condition effectively reduces friction 2. Slip flow enhances convective heat transfer
Period
• Friction over the bubble mattress may actually be substantial
References
• The Wenzel mode may induce larger friction reduction effects than the Cassie mode
1J.
P. Rothstein, Annual Review of Fluid Mechanics 42, 89-109 (2010). 2D. Maynes and J. Crockett, Journal of Heat Transfer, vol. 136, p. 011701, 2014. 3T. J. Kim and C. H. Hidrovo, Physics of fluids, vol. 24, pp. 112003-112003-18, 2012. 4T. J. Kim, R. Kanapuram, A. Chhabra, and C. H. Hidrovo, Transactions of the ASME-I-Journal of Fluids Engineering, vol. 134, p. 114501, 2012.
Graph of (a) sample of a velocity profile with slip flow near the wall and (b) the average slip length per period for varying penetration depth
• The Wenzel mode may be more efficient in removing heat than the smooth microchannel and the Cassie mode superhydrophobic microchannel
