Some Aspects of Drops Impacting on Solid Surfaces J.E Sprittles Y.D. Shikhmurzaev EFMC7 Manchester...
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Transcript of Some Aspects of Drops Impacting on Solid Surfaces J.E Sprittles Y.D. Shikhmurzaev EFMC7 Manchester...
Swansea 2007
Some Aspects of Drops Impacting on Solid Surfaces
Some Aspects of Drops Impacting on Solid Surfaces
J.E Sprittles
Y.D. Shikhmurzaev
EFMC7
Manchester 2008
Motivation
• Drop impact and spreading occurs in many industrial processes.
• 100 million inkjet printers sold yearly.
• Recent experiments show standard models of drop impact and spreading will be inadequate for microdrops.
Photos courtesy of Romain Rioboo
The Moving Contact Line Problem
dθU
Fails to provide a solution for a drop spreading steadily over a solid substrate.
The classical recipe of:
Bulk
Incompressible Navier Stokes
Solid
No-Slip
Free Surface
Stress balance with capillary pressure.
Flow Of Liquids Over Solids
Two Issues:– Allow For A Solution
– Describe The Angle Between The Free Surface and the Solid (The Contact Angle).
dθ U
Standard Solution:– Allow Slip Between
Solid and Liquid
– Let dθ = ( U )f
Standard Modelling of Axisymmetric Drop Impact and Spreading
• Bulk:
Incompressible Navier-Stokes
• Boundary:
Classical conditions apart from no-slip being replaced by Navier Slip:
• Contact Angle
Related to contact line speed using empirical relation (Jiang et al 79)
u.t n.P.t
Slip Tangential Stress
Finite Element Modelling:The Spine Method (Scriven and co-workers)
The Spine
Nodes fixed on solid.
Nodes define free surface.
Q) Does The Standard Model Work?Pyramidal Drops (Millimetre Size)
Experiment Renardy et al. Simulation Sprittles.
A) Qualitatively OKQuantitatively Not
ExperimentallyPrediction of
Standard Model
Standard Recipe’s Problems:
• Incorrect Kinematics
• Logarithmic Pressure Singularity at the Contact Line.
• But perhaps worst of all….
dθU
Can one describe the contact angle as a function of the parameters?
dθ ( U )f
U, m/s
dθ
Experiments answer, NO!
“There is no general correlation of the dynamic contact angle as a function of surface characteristics, droplet fluid and diameter and impact velocity.”
(Sikalo et al 02)
“There is no universal expression to relate contact angle with contact line speed”. (Bayer and Megaridis 06)
As in Curtain Coating (Used To Industrially Coat Materials)
Standard models:
Fixed Substrate Speed => Unique Contact Angle
U, cm/s
dθ
dθ
U
Dynamic contact angle as a function of coating speed for different flow rates (Blake & Shikhmurzaev 02).
Liquid Drops Spreading on Solids:Process of Interface Formation
lg
slSolid
Gas
Liquid
In Frame Moving With Drop
Interfaces are shown with finite thickness for representation
only.
The Interface Formation Model’s Predictions
Unlike conventional models:
• The contact angle is determined by the flow field.
• No stagnation region at the contact line.
• No infinite pressure at the contact line
=> Numerics easier
Simplest Model of Interface Formation
s1
*1
*1
s 1 11
s 1 111 1
1 1|| ||
v 0
n [( u) ( u) ] n n
n [( u) ( u) ] (I nn) 0
(u v ) n
( v )
(1 4 ) 4 (v u )
s se
s sss e
s
ff
t
p
t
s s1 1 1 2 2 2
1 3 2
v e v e 0
cos
s s
d
2
* 12 || ||2
s 2 22
s 2 222 2
12|| || || 2 22
21,2 1,2 1,2
u 1u 0, u u u
n [ u ( u) ] (I nn) (u U )
(u v ) n
( v )
v (u U ) , v U
( )
s se
s sss e
s s
s s
pt
t
a b
In the bulk:
On liquid-solid interfaces:On free surfaces:
At contact lines:
θd
e2
e1
n
n
f (r, t )=0 • Generalisation of standard/classical model
Qualitative Results:IFM In Frame Moving With Contact Line
lg
sl
Solid Speed U
Gas
Liquid
Qualitative Results:IFM In Frame Moving With Contact Line
lg
sl
Solid
Gas
Liquid
Solid Speed U
• Increasing impact speed• Changes flow field
Mock 05 et al - Drop Impact onto Chemically Patterned Surfaces
• Pattern a surface to ‘correct’ deposition.
Courtesy of Professor Roisman
Chemically Patterned Surfaces
• Flow over a transition between solids of differing wettabilities.
• Standard model predicts no effect.
1 2
2
Solid 1 Solid 2
What happens in this region?
Shear flow in the far fieldShear flow in the far field
Molecular Dynamics Simulations
Courtesy of Professor N.V. Priezjev
More wettable CompressedMore wettable CompressedLess wettable RarefiedLess wettable Rarefied
Results - Streamlines
Solid 2 less wettableSolid 2 less wettable
Qualitative agreementQualitative agreement
Sprittles & Shikhmurzaev, Phys. Rev. E 76, 021602 (2007).
Sprittles & Shikhmurzaev, EPJ (2008), In Print
Thanks
Drop Impact on a Hydrophobic (non-wettable) Substrate
Re=100, We=10, β = 100, .120s
Q) Does The Standard Model Work? Rebound on a Hydrophobic Substrate
Q) Does The Standard Model Work? Impact of a Microdrop
Radius = 25 m, Impact Speed = 12.2 m/sRe=345, We=51, β = 100, .67s
Experiment Dong 06. Simulation Sprittles.
Shikhmurzaev Model
• Solid-liquid and liquid-gas interfaces have an asymmetry of forces acting on them.
• In the continuum approximation the dynamics of the interfacial layer should be applied at a surface.
• Surface properties survive even when the interface's thickness is considered negligible.
Surface tension
Surface density
Surface velocity s
s
v
Shikhmurzaev ModelWhat is it?
• Generalisation of the classical boundary conditions.
• Considers the interface as a thermodynamic system with mass, momentum and energy exchange with the bulk.
• Used to relieve paradoxes in modelling of capillary flow.