Spontaneous Droplet Trampolining on Rigid Superhydrophobic Surfaces Schutzius, T
Spontaneous droplet trampolining on rigid superhydrophobic surfaces Schutzius, T. M. et al. Nature 527, 82–85 (2015).
Team 6: Ching Him Leung (Zeqian) Chris Li Kuan-Sen Lin Gengming Liu Outline
• Engineering the surface: lesson from nature
• Experimental set up and results
• Driving force for the bouncing: vaporization
• Modeling
• Droplet freezing
• Further studies
• Summary and critiques Hydrophobic/Hydrophilic/Superhdyrophobic
HydrophEngineeringobic Surfaces: “W theater-f esurfacearing surface” water Water tries to minimize contact with surface. hydrophobic surface Examples: Teflon, oily surfaces Hydrophobic/Hydrophilic/Superh•dSurfaceyroph morphologyobic and chemistry → different surface property Roughness Hydrophilic Surfaces: “Water-loving surface” water hydrophilic surface Hydrophobic Surfaces: “Water-fearing surface” water Water tries to maximize contact with surface. Water tries to minimize contact with suHrfaydce. rophobic/HydropExahilmpic/leSus: Gplaess,rh rudstyroed mepthalo subrfiacce s • Liquidhydrop droplethobic surf behaviorace on the surface Examples: Teflon, oily surfaces
water Hydrophobic Surfaces: “Water-fSueapriengrh suydrfroacep”h obic Surfacewatse:r Hydrophobic Hydrophilic Surfaces: “Water-loviWnagt esur trfriaeces t”o minimize contawcta tweri th surface. Water tries to maximize contact with surface. hydrophilic surfsuacerfa ce having nano-scahydlero rophuogbhicn esuss.rfa ce superhydrophobic Examples: Teflon, oily surfaces Examples: Glass, rusted metal surfaces • Application: ex. Self-cleaning
Hydrophilic Surfaces: “Water-loving surface” water Water tries to maximize contwactat ewr ith surface. hydrophilic surface Superhydrophobic Surfaces: Hydrophobic Examples: Glass, rusted metal surfaces surface having nano-scale roughness. superhydrophobic
water Superhydrophobic Surfaces: Hydrophobic surface having nano-scale roughness. superhydrophobic Nature-Inspired Superhydrophobic Surfaces Nature-Inspired Superhydrophobic Surfaces
Can learn from Nature Double roughening of a hydrophobic surface, on the Dsuoubbmile crorougnh aennidn gn oafn ao mehydtroepr hobic • Lotus leaf scale, creates susuprfeacerh,yd onro thpeh osubbicimity!cro n and nanometer scale, creates superhydrophobicity!
Zoom in
A water droplet beads Au pw aotner ad roloptulest beads up on a loSEMtus images of SEMlotus i maleagfe sus orff alocetus leaf surface leaf due to the hydroplheoafb dicu e to the hydrophobic nanostructures nanostructures• When the leaf tilts a little bit the droplet will roll off.
http://www.pbase.com/yvesr Cheng, Y. T., et. al. AppCheng,lied Physi Y.cs T.,Le tet.ters al.2005, Applied 87, 1941 Physics12. Letters 2005, 87, 194112. http://www.pbase.com/yvesr Cheng, Y. T., et. al. Applied Physics Letters 2005, 87, 194112. Droplets levitate due to vaporization
https://commons.wikimedia.org/ wiki/File:Leidenfrost_droplet.svg http://www.irishmanabroad.com/2015/02/scien ce-saturday-leidenfrost-effect-explained/ In the work we are presenting…
• Combine super-hydrophobic surface + vaporization effect.
• Don’t rely on hot plate to create vapor.
• In special environment condition, single droplet can jump spontaneously. Experimental set up and results Setups and conditions
• Low ambient pressure (0.01 bar)
• Low environmental humidity
• Controllable water droplet sizes
• Silicon micropillar specifications Spontaneous Trampolining Characterizing Motion Hunting key Forces
Low pressure case
Normal Pressure case Continued
• Measure restitution coefficient 풗 휺 = ퟐ 풗ퟏ • Assume invariant mass to estimate the momentum change
∆푡푟 ሚ ∆푃 = න 푓 푑푡 ≈ 푓∆푡푟 0 • Then estimate average force Continued
• Then estimate average force
∆푃 ሚ 푓 = ≈ 2.2 휎푅0 ∆푡푟
푓ሚ 휎푅0 • Pressure = = 2.2 × 2 퐴푟푒푎 휋푅푀푎푥 2휎 • Pressure ≈ 0.9 × ( ) 푅0 Vaporization Can vaporization account for the driving force
• Vapor flow generate the force needed to accelerate the droplet • At low pressure
high vaporization flux 퐽
vapor can be trapped between the micropillar
Excess pressure between the droplet and the surface Measuring vaporization flux
• At pressure lower than 0.1bar, vaporization flux increases significantly Overpressure due to vaporization
• Overpressure Δ푃 ∝ 퐽
• Using the measured vaporization flux, the maximum overpressure Δ푃 ≈ 4.7(2휎/푅)
• Since the pressure varies between 0 and 4.7(2휎/푅), the average pressure is ΔPഥ ≈ 2.3(2휎/푅)
• From previous slides, ΔPഥ ≈ 0.9(2휎/푅0) 푅 is the contact radius 푅0 is the radius of the droplet Modeling Modeling
• Toy model: forced, mass-spring-damper system.
1 푦 = 푦 + 푣 푡 − 푔푡2 (free fall) 푖푓 푦 ≥ 0 0 0 2
푑2푦 푑푦 푚 + 푐 + 푓 푦 = 푓 푡 − 푚푔 푖푓 푦 < 0 푑푡2 푑푡 푘
푦 = 0 Spring: The droplet deforms like a spring when it impacts the surface. Modeling
• Toy model: forced, mass-spring-damper system.
Blue: experiment Black: modeling 1 푦 = 푦 + 푣 푡 − 푔푡2 (free fall) 푖푓 푦 ≥ 0 0 0 2
푑2푦 푑푦 푚 + 푐 + 푓 푦 = 푓 푡 − 푚푔 푖푓 푦 < 0 푑푡2 푑푡 푘
Spring: The droplet deforms like a spring when it impacts the surface. Droplet freezing Droplet freezing and levitation
Strong vaporization
Supercools to -20°퐶
Recalescent freezing • Sudden temperature increase: -20°퐶 → 0°퐶 • Sudden latent heat release
Explosive flux
Aluminum surface, 24°퐶, ~0.01bar, humidity<10% Levitate Droplet freezing and levitation
Strong vaporization
Supercools to -20°퐶
Recalescent freezing • Sudden temperature increase: -20°퐶 → 0°퐶 • Sudden latent heat release
Explosive flux
Silicon micropillar surface; 25°퐶, humidity<10% Levitate Further studies
• Hydrogel • Heated surface
• Initial jump: massive water loss • Trampolining after: same mechanism
Pham, J. T., Paven, M., Wooh, S., Kajiya, T., Butt, H. J., & Vollmer, D. (2017). Spontaneous jumping, bouncing and trampolining of hydrogel drops on a heated plate. Nature communications, 8(1), 905. Summary How is this not violating the second law?
Trampolining Resonating force
High Flux trapped in vaporization surface structure
Allowed by second law! Our critiques
• Pressure control? Our critiques
• Pressure control? • The estimation of ΔPഥ ≈ 2.3(2휎/푅) is too rough
• 0.9(2휎/푅0) and 2.3(2휎/푅) are not consistent as the authors claimed. • It does not contain enough experimental trials to convince readers that the shown contact radius dynamics is a good representation of the general trend. Thanks!