1. Interfacial Rheology System

2. Background of Interfacial Rheology
Interfacial Shear Stress  Interfacial Shear Viscosity 
 =    [] = Pa·s·m = N·s/m or surface poise
„Ability of a liquid interface to transport momentum in shear deformation within its own plane “
air
Interface
surface velocity gradient
water
oil
Bulk liquid
water
Boussinesq Number

3. Background of Interfacial Rheology
Geometry Used: Bicone
With the cone located directly at the liquid / liquid or the liquid / air interface
2-dimensional Couette System
Interface flow is coupled to bulk phase flow
Analysis of the flow field for the Bi-Conical Disk Rheometer for taking care of contributions from liquid 1 and liquid 2:
(Oh & Slattery (1978) J.Colloid Interface Sci. 67(3): )

4. Positioning of the Bicone
Double distilled water is filled into the cup of the Interfacial Rheology System.
The Bicone is positioned at the water / air interface by an Normal force assisted technique:

5. How to Create an Interfacial Film
A material with a hydrophobic and a hydrophilic part forms a layer on a water surface.
!!! Only if a film is present interfacial shear rheology makes sense !!!
Two methods to create interfacial films:
1. Spread Films
mainly for low molecular weight surfactants
dissolving in a spreading solvent (e.g. hexane, ethanol, chloroform)
spreading directly onto the water with a micro-syringe
waiting for complete evaporation of the solvent
pouring the oil phase on top of the surfactant film
2. Absorbed Films
for example for interfacial layers of protein
dissolving the proteins in distilled water
pouring the oil phase gently on top of the water / protein solution
films are created by absorption from the bulk phase to the interface

6. Building of a Protein Film at the Water/Oil Interface
Due to their large size protein molecules diffuse slowly from solution to the surface
At the surface they build a network, but this is also a time consuming process
Network building can be accelerated by heating the solution before the experiment
Heated protein molecules modify their structure, which leads to a better adsorption at
the interface and a network building trough cross linking of the amino acids.

7. Flow Curve at the Oil/Water Interface
Sorbitan tristearate (Span 65) at the oil/water interface

8. Film Formation of a Coffee Sample at Different Concentrations
0.1% strain, frequency 1Hz
0.05g, 0.15g, and 0.3g coffee powder / 114ml double distilled water

9. Amplitude Sweep on a Coffee Film Compared to Pure Water / Air Interface
Concentration of the coffee film: 0.3g /114ml
Frequency: 1 Hz
Limit at 0.3 µrad in deflection angle and 3 nNm in torque !

10. Competition
De Nouy Ring
Bicone
Double Wall Ring

11. The Bicone Advantages Disadvantages
Robust well defined measuring system
Complete flow field analysis for calculation of interfacial values
Use of large gap cylinder analogy
Works on liquid/liquid and liquid/air interfaces
Accurate positioning over normal force sensor
Established measuring system for interfacial rheology with numerous references
Drag cup rheometers can not handle this geometry due to the high mass and high moment of inertia
Low and high interfacial viscosities measurable
Stainless Steel Bicone
RO = 40 mm, Ri = 34 mm
Disadvantages
Large sample volume required
Low Boussinesque number

12. De Nouy Ring Advantages Disadvantages
Simple, light geometry known form surface tension measurements
Measuring system that drag cup rheometer can handle due to its low moment inertia
Simple concentric cylinder geometry analogy for calculation
MCR can also measure with the De Nouy Ring
Small sample volume
Platinium-Iridium (Pt-Ir)
Wire diameter = 0.36 mm
Re = 40 mm, Ri = 10 mm
Disadvantages
No flow field analysis available
Film flow can not be separated from subphase coupling
Fragile measuring system
Accurate positioning is difficult due to wetting properties of the ring
Not suitable for very high interfacial viscosities
Scientific interfacial community came to the conclusion:
„Rings are for fingers but not for serious interfacial rheological measurements“

13. Comparison of Amplitude Sweeps (1Hz) Bicone (red) – Du Nouy Ring (blue)

14. Comparison Frequency Sweep Bicone (red) – Du Nouy Ring (blue)
Bicone (0.01% strain)
Du Nouy (0.1% strain)
Bicone is one decade more sensitive and allows 3 times higher frequencies.

15. Closer Look on Double Wall Ring Data Proceedings ISFRS 2009 / Rheol Acta 2009
The contact area with the subphase/covering phase is reduced for the Double Wall Ring geometry compared to the Bicone resulting in a higher Boussinesque number.
Therefore interfacial measurements should be theoretically possible at films with lower structural strenght.
Boussinesque number:
Indeed the bousinesque number is higher for the DWR compared to the
Bicone, but the Bicone employs an accurate correction for the subphase drag.

16. Comparison of Two Data Sets of TA / KU Leuven Proceedings ISFRS 2009 / Rheol Acta 2009
time
Time Sweep of lysozym protein (Absorbed film, film builds up over time)
Over time the interface changes from pure water/air to a protein network/air interface (phase angle from 90° to ~10°)
N1 to N5 are different needles for the interfacial needle rheometer, which was used as reference.
Strain Sweep of Span65 (Spread film)
Base line determines the lower border for interfacial measurements
Base line (water/air interface) inertia dominated  G´is measured

17. Coments to the Data Span 65
Base line (pure water/air interface) inertia dominated GI` ~ 4x10-5, GI´´ ~ 10-5,
G*I`~ 4,6x10-5
Lysozym
It is stated that the Bicone can measure from Gi*~ 10-4 which is roughly the correct value but the measurement with the DWR has just two points more. These two points can be found at a phase angle of 90°. Between a phase angle of 70° to 90° or Gi* between 10-4 and 10-5 Pa*m respectively there is not a single point given by the DWR.
Theoretically this area should be the strenght of the DWR as the advantageous Boussinesque number should allow to collect data here.
At the smaller phase angles the Bicone and needle rheometer show similar results whereas the phase angle for the DWR is much higher. Are there Compliance problems of the ring?
The Bicone can measure up to the highest interfacial viscosity values.

18. Bicone: Film Formation of Instant Coffee
0.1% strain, frequency 1Hz
0.05g, 0.15g, and 0.3g coffee powder / 114ml double distilled water
Gi´= 3*10-5 Pa*m

19. Strain Sweep Bicone Span65 at the air/water interface
The measurements shown in proceedings for the ISFRS 2009 on Span65 with the Double Wall Ring (DWR) geometry have been reproduced with an MCR 301 and the Bicone measuring system for comparison.
Min torque:
3 nNm
Water base line

20. Comparison Bicone / Double wall ring Strain Sweep
~ 0.4 molecules/nm2
~ 1 molecule/nm2
~ 4
molecule /nm2
Water base line

21. Frequency Sweep Bicone Span 65 at the air/water interface
Water/air base line indicates the measuring limit as the fluid inertia is dominant.

22. Comparison Bicone / Double Wall Ring Frequency Sweep
~ 0.3 molecules/nm2
~ 1 molecule/nm2
~ 4
molecule /nm2
Min. torque 6 nNm

23. Active Moment of Inertia Compensation vs. Low Inertia due to Low Mass
Mass Bicone (BC)
~ 103g
Mass Double Wall ring (DWR)
Approx. 15g
Water/air base line
Freq. Limit
BC: 10 rad/s
DWR: 10 rad/s
Freq. Limit
BC: 10 rad/s
DWR: 5 rad/s
Freq. Limit
BC: 3rad/s
DWR: 1 rad/s

24. Conclusions Double Wall Ring vs. Bicone
The theoretical advantage of using the Double Wall Ring with a smaller
Boussinesque number, could not be shown in measured data until today.
The interfacial analysis of the Bicone allows a very accurate correction for
the subphase influence despite of the higher Boussinesque number.
Data produced with the Double Wall Ring on Span65 could be reproduced
with the Bicone.
Due to the active inertia compensation the interfacial properties could be
measured up to the intrisic frequency limit of the interface (Fluid/interface
inertia) despite the mass of the Bicone is approx. 10 times higher then the
DWR.