A new interpretation for the dynamic behavior of complex fluids at the sol-gel transition using the fractional calculus

Transcription

1 A new interpretation for the dynamic behavior of complex fluids at the sol-gel transition using the fractional calculus By Stephane Warlus and Alain Ponton Rheol. Acta (009) 48:51-58 Chris Dimitriou NNF Summer Reading Group 7/1/010 1

2 The Critical Gel Terminology Liquid Solid Transition (LST) Sol-Gel Trasition (SGT) Critical Gel For a polymer, as crosslinking increases, the material approaches a gel Idea of p p is the ratio of the number of chemical bonds to the total number of possible bonds * As RH we have a critical gel, I.e. the whole network is interconnected p = p c Idea can be extended to physical gels not restricted to crosslinking in polymers *Winter and Mours, Rheology of Polymers near Liquid- Solid Transitions, Advances in Polymer Science, 1997

3 When do we have a critical gel? The material transitions from behaving like a fluid (relaxes completely) to a solid (has a non zero equilibrium modulus) Relaxation Modulus, G(t) p increasing Viscoelastic Moduli n Phase Angle Relaxation modulus Winter & Mours Warlus & Ponton (p,t) G(t) St n G(t, p c ) S pc t pc t G(t t ') & (t ')dt ' *Winter and Mours, Rheology of Polymers near Liquid- Solid Transitions, Advances in Polymer Science,

4 Contribution of Warlus & Ponton Use of Riemann-Liouville integral operator to solve for constitutive model of a critical gel: (p,t) t G(t t ') & (t ')dt ' t (p c,t) S pc (t t ') pc & (t ')dt ' G(t, p c ) S pc t pc Fractional Element (spring pot) Behavior intermediate between elastic solid and Newtonian fluid, described by following: pc S pc (1 pc ) D f (t) 1 () D f (t) d f (t) dt t 0 f (t ') dt ' 1 (t t ') (p c,t) S pc (1 pc ) d pc (t) dt pc Fractional Element! 4

5 What does the springpot predict for G, G and tan()? As expected, a power law relaxation modulus: G FE (t) (1 ) t G,G and tan() are given by: d sint 0 dt 0 sin t G '() cos G ''() sin tan() tan 5

6 Friedrich vs. Warlus Different ways to extract viscoelasti moduli Friedrich, Schiessel and Blumen: E d (t) dt G * () %() / % () %() E d (t) dt e it dt E (i) G * () E (i) ; where: E Multiplication rule % () Warlus and Ponton: d dt sint 0 0 sin t G '() cos G ''() sin 6

7 The Post-SGT State Warlus and Ponton argue that the behavior in the post SGT state can be described by the fractional Kelvin-Voigt model (KVF) G ep p, p G ep p Shear stress relationship: KVF (p,t) p, p d p (t) dt p G ep (t) Shear stress relaxation modulus: G KVF (p,t) p, p (1 p ) t p G ep 7

8 The Pre-SGT state Although not written in their 009 paper, the authors have used expressions for the relaxation modulus in the pre SGT state Extended relaxation modulus expression*: G(p p c,t) S p t p exp(t / p ) G ep Pre SGT: CG: Post SGT: G(p p c,t) S p t p exp(t / p ) G(p p c,t) S pc t pc G(p p c,t) S p t p G ep This expression allows the parameters to have relevance in regions not directly at the SGT *Warlus et. al., Eur. Phys. E, 003 8

9 The fractional derivative order and fractal dimension Warlus states that dynamic light scattering (DLS) experiments show power law relaxation in the dynamic structure factor of some systems Structure and the critical exponent can be related as follows: M : R d f M Molecular Mass of clusters d d d f d d f M. Muthukumar, Macromolecules 1989 R d f d Size of clusters Fractal Hausdorff Dimension Dimension of space (3d space) The fractal dimension can be understood by considering the Koch curve: Box counting is a common method used to determine the fractal dimension of a particular shape 9

10 Connection to physical gels: wax-oil mixtures Wax-Oil gels are formed when wax precipitates out of the solution at lowered temperatures. A clear gelation point can be identified A 5% wax in oil mixture: T wa =30 C 10s % 1s 1 T gel =5.5 C 0.4mm T=7 C T=7.5 T=8 C 0 0.1% C 0 0.1% 0 0.1% n ~

11 Relating slope of G () to fractal dimension Warlus and Ponton (Rheol. Acta 009) give the following relation: n d d d f d d f Where d f is the fractal dimension Study by Gao (J Phys. Cond. Matter 006) 10% Wax Oil System (5 C): d f

12 Box Counting Algorithm The box counting algorithm is an simple algorithm to implement so as to determine the fractal dimension from digital images Begin with raw image: Threshold/Extract boundary: Box Size With boxes of varying size (), measure how many are required to cover the boundary (N boxes) Plot log(n) vs log() and the slope is equal to the box counting fractal dimension 1

13 Conclusion Critical gel: Terminology and behavior Using fractional calculus and fractional elements to understand behavior at the critical gel point Viscoelastic moduli, relaxation modulus Pre and post SGT state according to Warlus and Ponton Connection between fractional derivative order and fractal dimension Connecting Warlus work with a physical gel Thank you for your attention! Questions? 13