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Soft Matter : Routes To Rheochaos, Anomalous Diffusion And Mesh Phases

Electronic Theses of Indian Institute of Science

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Title Soft Matter : Routes To Rheochaos, Anomalous Diffusion And Mesh Phases
 
Creator Ganapathy, Rajesh
 
Subject Diffusion
Material Science
Mesh Phases
Rheochaos
Wormlike Micellar Gels - Dynamics
Cetyltrimethylammonium Tosylate (CTAT)
Soft Condensed Matter Systems
Nonlinear Dynamics
Soft Matter
Wormlike Micelles
Soft Condensed Matter (SCM)
Materials Science
 
Description Soft condensed matter (SCM) systems are ubiquitous in nature. SCM systems contain mesoscopic structures in the size range 10 nm to 1 am that are held together by weak entropic forces. These materials are therefore easily perturbed by external
fields such as shear, gravity and electric and magnetic fields and are novel systems
for studying non-equilibrium phenomena. The elastic constants of these materials are
≈ 109 times smaller than conventional atomic fluids and hence it is possible to measure the viscoelastic response of these materials using commercial instruments such as
rheometers. The relaxation time in SCM systems are of the order of milliseconds as
compared to atomic systems where relaxation times are of the order of picoseconds. It
is easy to study the effect of shear on SCM, as the shear rates attainable by commercial rheometers are of the order of the inverse of their relaxation times. The dynamics
of SCM systems and their local rheological properties obtained using the method of probe diffusion can be quantified through dynamic light scattering experiments. The
structure of SCM systems can be quantified using diffraction techniques such as small
angle x-ray scattering. In this thesis we report experimental studies on the linear
and nonlinear rheology and the dynamics of surfactant cetyltrimethylammonium tosylate (CTAT), which forms cylindrical wormlike micelles, studied using bulk rheology and dynamic light scattering (DLS) technique, respectively. We have also studied the phase behaviour of the ternary system formed by cetyltrimethylammonium 3-hydroxy-napthalene 2-carboxylate (CTAHN), sodium bromide (NaBr) and water using small angle x-ray scattering (SAXS).

In Chapter 1, we discuss why SCM systems are suitable for studying non-equilibrium
phenomena such as the effect of shear on the structure and dynamics of condensed matter. This is followed by a discussion on the chemical structure, phase behaviour and self assembling properties of the amphiphilic molecules in water. We then discuss the intermacromolecular forces such as van der Waals interaction, the screened Coulomb repulsion and hydrophobic and hydration forces. The systems that have been the subject of our experimental studies, viz. CTAT and CTAHN/NaBr/water have also been
discussed in detail. This is followed by a theoretical background of linear and nonlinear rheology, dynamic light scattering and small angle x-ray scattering techniques. Next we describe the stress relaxation mechanisms in wormlike micelles. This is followed by a discussion on some standard techniques of nonlinear time series analysis, in particular the evaluation of the delay time L, the embedding dimension m, the correlation dimension ν and the Lyapunov exponent λ. We have also mentioned a few examples of experimental systems where chaos has been observed. We have also discussed in detail the various routes to chaos namely, the period-doubling route, the quasiperiodic route and the intermittency route. The concluding part of this chapter summarises the main results of the thesis.

Chapter 2 discusses the experimental apparatus used in our studies. We have
discussed the different components of the MCR-300 stress-controlled rheometer (Paar
Physica, Germany). The rheo-small angle light scattering experiments and the direct
visualisation experiments done using a home-made shear cell are also discussed. Next
we describe the various experiments that can be done using a commercial rheometer. The frequency response and flow experiments have been discussed with some examples from our own work on entangled, cylindrical micelles. This is followed by a discussion on the various components of our dynamic light scattering (DLS) setup (Brookhaven
Instruments, USA). Particle sizing of submicrometer colloidal spheres using our DLS
setup has been discussed with an example of an angle-resolved DLS study of 0.05µm
polystyrene colloids. Next we describe the various components of the SAXS setup
(Hecus M. Braun, Austria). As an example application of SAXS we have quantified
the structure of the lamellar phase formed by the surfactant CTAHN/water. We finally
describe the sample preparation methods employed by us for the different experiments.

Our nonlinear rheology experiments on viscoelastic gels of surfactant CTAT (cCT AT=
2wt%) in the presence of salt sodium chloride (NaCl) at various concentrations has
been discussed in Chapter 3. We observe a plateau in the measured flow curve and this is attributed to a mechanical instability of the shear banding type. The slope of this plateau can be tuned by the addition of salt NaCl. This slope is due to a concentration difference between the shear bands arising from a Helfand-Fredrickson mechanism. This is confirmed by the presence of a “Butterfly” light scattering pattern in SALS experiments performed simultaneously with rheological measurements. We have carried out experiments at six different salt concentrations 10mM < cN aCl
 
Contributor Sood, Ajay K
 
Date 2008-10-06T05:03:55Z
2008-10-06T05:03:55Z
2008-10-06T05:03:55Z
2006-09
 
Type Thesis
 
Identifier http://hdl.handle.net/2005/372
 
Language en_US
 
Rights I grant Indian Institute of Science the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation.