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!