M. Coletti
TA Instruments Italy
Rheology of structured fluids
TA Instruments - a brief introduction
TA Instruments (a subsidiary of Waters ltd.) is the worldwide leader in Thermal Analysis, Rheology and Microcalorimetry equipment
TA Instruments is a manufacturer based in New Castle (Delaware) but has direct offices in more than 20 countries worldwide providing: Sales Application Support Service Support
TA has a long story in Rheology (in 1993 acquired Carri-Med and in 2003 Rheometrics)
Rheology equipment
Why characterize rheology of SF?
Structured fluids are everywhere (foods, cosmetics, pharmaceuticals, coatings)
Emulsions, dispersions and foams (as well as multiphase systems) are often inherently unstable
Their stability (or the strategies to improve their stability) is related to the viscosity of the liquid matrix and the interactions among the particles and between the particle and the matrix
From a rheological point of view, several relationships between viscosity and shear rate/stress are possible: Bingham, Shear thinning, Pseudoplastic, Shear thickening, Thixotropy
Example: Plastic flow + dilatancy
Two "unusual" characterisations
Laminar flow in the liquid matrix can have an effect on the "structure" of the dispersed phase Small Angle Light Scattering (SALS) can study the orientation of
particles
Unstable dispersions can be studied from an interfacial point of view Double Wall Ring (DWR) is an exclusive accessory to perform
interfacial rheology studies
SALS
A laser beam is shun through a sufficiently transparent dispersion
Dispersed particles (diameter ranging from 1 to 4.6 micron) scatter the light and the resulting scattering pattern is collected by a camera
When (if) the particles are oriented by the laminar flow, Bragg's law can be used to work back the orientation pattern
A
B
C
Incidentrays
Reflectedrays
φd
φ φ
d sinφ
Shear-Induced crystallization
Non-Equil. Configuration Under Shear Crystalline State
γ = 0 γ ≠ 0 γ = 0
Sample: Silica particles (1 micron - monodispersed) concentrated in PEG
Test method: Large Amplitude Oscillation (LAOS) at 100% or more of shear strain
Shear-Induced Crystallization
0 200 400 600 8000.00
0.04
0.08
0.12
0.16
0.20
0
2
4
6
8
Af
Af
(6 f
old
)
Time (sec)
τ
τ (P
a)
DWR and interfacial rheology
The properties at the interface between two liquids or a liquid and air are strongly affected by the presence of a dispersed phase
If segregation occurs at the interface, rheology can provide useful information on the kinetic of the phenomenom
Double Wall Ring (DWR) is an innovative geometry that sums all the benefits of previous systems such as DuNouy Ring and Bicone
Interfacial shear
Structure of the interface
Sample: CTAB (Cetyltrimethylammonium bromid) in water vs. n-decane
1E-4 1E-3 0.01 0.1 11E-6
1E-5
1E-4
1E-3
1E-3
0.01
0.1
1T=25oCω=0.5 rad/s
Torque limit
CTAB 0.01M in water at n-decane interface
Sto
rag
e &
Lo
ss M
od
ulu
s [N
/m]
G' s,
G" s
Strain γ [ ]
To
rque
[µNm
] M
Kinetics of segregation
Sample: Aβ42 prion - insoluble protein in water vs. n-decane
Conclusions
As structured fluids have complex rheological behaviours, it is important to have different approaches and techniques of characterisation
Apart "standard" characterisation (flow curves, LVR, creep) other properties can be assessed through hyphenated techniques (SALS) and specific geometries (DWR)
Grazie!
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