Huamin Wang, Peter J' Mucha and Greg Turk

1
Water Drops on Surfaces

• Huamin Wang, Peter J. Mucha and Greg Turk
• Georgia Institute of Technology
• From SIGGRAPH 2005
• Presented by Huamin Wang (whmin_at_cc.gatech.edu)

2
Motivation

• Film special effects

3
Motivation

• Scientific Visualization

4
Motivation

• Game industry (eventually)

5
Motivation

• Game industry (eventually)

6
Overview

• Background
• The virtual surface method
• Dynamic contact angle model
• Results and Analysis
• Future work

7
Background

• Small-scale liquid-solid Interactions

Q Why small water behaves naturally different
from large water? A1 Surface Tension (water
72 dynes/cm at 25º C). A2 Viscosity (water
1.0020 10-3 Ns/m2 at 20º C).
Lake view ( gt1 meter)
Water drops (in millimeters)
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Background

• To calculate surface tension force
• Tension coefficient (always positive)
• Mean curvature
• Normal (always pointing outward)

(Laplaces Law)
Uniform curvature
Water sphere photo taken on the International
Space Station. Courtesy NASA
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Background
Stable contact angle satisfies Youngs Relation
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Background

• Our work is based on fluid simulation using
  Computational Fluid Dynamics (CFD).
• Solve the Navier-Stokes equations for the
  velocity field
• Use the particle Level-Set method

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Overview

• Background
• The virtual surface method
• Dynamic contact angle model
• Results and Analysis
• Future work

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The virtual surface method

• Solution
• Place a virtual surface beneath the solid plane
• Estimate the surface tension using the new
  combined surface

• Problem
• Real World a stable contact front with contact
  angle
• Simulation Curvature at contact front is
  positive, thus always pushes inward

Air
Liquid
Solid
Virtual Liquid
Virtual Surface
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The virtual surface method

• Create a virtual surface
• Estimate curvature on the contact front
• A kink cause the curvature to push the fluid
  front

Air
Air
Liquid
Virtual Liquid
Virtual Liquid
Solid
Solid
Virtual Surface
Virtual Surface
Advancing to right
Receding to left
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The virtual surface method

• Details? Please read our paper.

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Overview

• Background
• The virtual surface method
• Dynamic contact angle model
• Results and Analysis
• Future work

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Dynamic contact angle model
Contact angle hysteresis Small water drops can
stay on a vertical plane, while large water drops
will flow down.
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Dynamic contact angle model

• Stable contact angles bounds
• Advancing
• Receding
• a valid stable contact angle

• Final pressure P is

Pa pressure calculated using Pr
pressure calculated using
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Dynamic contact angle model

• Dry/Wet conditions
• wetting history map
• Contact angles based on surface wetness
• (wet advancing angle smaller than the dry
  advancing angle)

Dusted region is dry, transparent region is wet.
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Overview

• Background
• The virtual surface method
• Dynamic contact angle model
• Results and Analysis
• Future work

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Results and Analysis

• Physics phenomena

Capillary Action
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Results and Analysis

• Drop impacts, rivulets, dripping drops, and more

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Results and Analysis

• Drop impacts, rivulets, dripping drops, and more

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Results and Analysis

• Video

(playback speed 10 times slower than real world
speed)
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Results and Analysis

• Grid resolution 400400400
• Simulation speed 20 minutes per frame
• Each sequence has 500 frames
• The total running time is
• 20 500 / (60 24) 7 days

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Results and Analysis

• Why is it relatively computational expensive?
• High grid solution (400400400)
• Large viscosity effects
• Implicit Euler method
• The condition number of the linear system
  increased.

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Overview

• Background
• The virtual surface method
• Dynamic contact angle model
• Results and Analysis
• Future work

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Future Work

• Octree data structure
• Virtual surface reconstruction based on particles
• Distributed computing

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Acknowledgements

• We would like to thank
• Mark Carlson, Chris Wojtan, Howard Zhou, Spencer
  Reynolds, Nathan Sisterson
• Everyone supporting our work, including
  reviewers.
• Gatech Computational Perception Laboratory,
  Geometry Group
• CMU graphics lab
• Funded
• In part by NSF grant DMS 0204309.
• Rendering
• Physically Based Ray tracer (pbrt), Matt Pharr
  Greg Humpheys
• Light Probe Image Gallery, Paul Debevec

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Fin Any questions?
Water Drops on Surfaces
Huamin Wang, Peter J. Mucha and Greg Turk Georgia
Institute of Technology From SIGGRAPH
2005 Presented by Huamin Wang (whmin_at_cc.gatech.edu
)