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
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 iscoupled 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): 516-525)
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:
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
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.
Flow Curve at the Oil/Water Interface Sorbitan tristearate (Span 65) at the oil/water interface
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
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 !
Competition De Nouy Ring Bicone Double Wall Ring
The Bicone Advantages • 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
De Nouy Ring Advantages • 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) Wirediameter = 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“
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.
Closer Look on Double Wall Ring DataProceedings 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.
Comparison of Two Data Sets of TA / KU LeuvenProceedings ISFRS 2009 / Rheol Acta 2009 ISFRS Rheol. Acta 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
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.
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
Strain Sweep BiconeSpan65 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
Comparison Bicone / Double wall ringStrain Sweep ~ 0.4 molecules/nm2 ~ 1 molecule/nm2 ~ 4 molecule /nm2 Water base line
Frequency Sweep BiconeSpan 65 at the air/water interface Water/air base line indicates the measuring limit as the fluid inertia is dominant.
Comparison Bicone / Double Wall RingFrequency Sweep ~ 0.3 molecules/nm2 ~ 1 molecule/nm2 ~ 4 molecule /nm2 Min. torque 6 nNm
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
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.