Physics (PHY)https://etd.iisc.ac.in/handle/2005/502024-03-29T15:55:26Z2024-03-29T15:55:26Z1H-NMR Study of Proton Glasses - Nuclear Spin Lattice Relaxation in BPxBPI(1-x) and BPxGPI(1-x) - Effects of Disorder on the Proton Group DynamicsRamanuja, M Nhttps://etd.iisc.ac.in/handle/2005/46932020-12-04T04:50:46Z1H-NMR Study of Proton Glasses - Nuclear Spin Lattice Relaxation in BPxBPI(1-x) and BPxGPI(1-x) - Effects of Disorder on the Proton Group Dynamics
Ramanuja, M N
Mixed systems made from a combination of ferroelectric (FE) and antiferroelectric (AFE) compounds, exhibit various effects of disorder in different temperature regions. The kind of effects observed, depend on the technique and the window of observation employed. Model systems, like Potassium Ammonium dihydrogen phosphate (KADP), Rubidium Ammonium dihydrogen phosphate (RADP) and BPxBPI(1-x) , with H-bonding networks, have been well studied by dielectric techniques. These investigations have revealed disorder effects like deviations from Curie Weiss law, progressive broadening of dielectric loss curves and dispersion of dielectric constant, at sufficiently low temperatures. NMR studies in such systems are meager and mainly members of the KDP family, like Rubidium ammonium dihydrogen phosphate (RADP) and arsenate (RADA) have been investigated using mainly 2H and 87Rb NMR. On the other hand, proton NMR has been much less used, and our focus is to exploit its power/potential to study 1H group dynamics in the presence (and absence) of disorder in condensed matter systems.
This thesis describes the results of proton NMR investigations in two mixed systems of ferroelectric and antiferroelectric compounds namely, (i) Betaine phosphate (BP, AFE) and Betaine phosphite (BPI, FE) and (ii) Betaine phosphate and Glycine phosphite (GPI, FE). The aim of the study is to obtain information on 1H group dynamics (activation energies and pre-exponential factors) and the effects of micro-spatial disorder. The former system is shown to exhibit orientational glass behavior by extensive dielectric investigations. BP-GPI system is synthesized for the first time and our proton NMR investigation has exhibited interesting effects of disorder like deviation from expected BPP behavior. Further, both systems have exhibited quantum tunneling effects, revealing a gradual transition from classical regime to quantum regime. Biexponential magnetization recovery at low temperatures has also been observed indicating the existence of disorder.
A combination of AFE and FE compounds of this type form a mixed system, over a broad range of compositions, in which the long-range electric order is suppressed owing to frustration effects. Such systems have been treated as dipolar analogues of spin glasses and are known as ‘orientation glasses’ (OG), ‘proton glasses’ (PG) or ‘pseudo-spin glasses. Although the frustrated condensed matter system is crystalline in nature, there is an underlying microstructural randomness due to local fluctuations of the composition which usually results in static lattice strains, which are called random fields. It has been shown that these random fields can also have a pronounced effect on the spin lattice relaxation time as observed in NMR experiments. Depending on the relative concentration and temperature, the mixed system exhibits a range of states (x-T phase diagram) like FE, OG, coexisting OG and AFE, and AFE.
These mixed systems exhibit various kinds of effects of disorder in different temperature regimes which depend upon the technique and window of observation. For e.g., using dielectric spectroscopy we can study the behavior of the electric dipoles during various phases and the effects of frustration seen as dispersion of dielectric constants and broadening of loss curves etc. Through quadrupole perturbed NMR study of systems containing nuclei like 87Rb or 2H, we learn about site-specific inhomogeneities and distribution of EFG in the system. Proton NMR study in the mixed systems, though not much used so far, is a powerful technique to shed light on the dynamics, disorder and Quantum tunneling effects.
Our proton SLR time measurements have been carried out at two Larmor frequencies of 23.3 MHz and 11.4 MHz, in the temperature range of 300 K to 4 K and the results are presented in this thesis, which is divided into four chapters
3D Dosimetry using optical tomography and electronic portal imagesManjappa, Rakeshhttps://etd.iisc.ac.in/handle/2005/44842023-02-09T06:22:34Z3D Dosimetry using optical tomography and electronic portal images
Manjappa, Rakesh
The primary goal of this thesis is to develop techniques to quantify the radiation
dose distributions used in radiotherapy for cancer treatment. It aims at developing
a physically profound calculation model for the transit dosimetry by a detailed
characterization of the radiation interaction with tissues and the
fluence
measurements recorded in electronic portal imaging device (EPID). Radiotherapy
has undergone great advances with developments such as Intensity Modulated
Radiotherapy (IMRT), Volumetric modulated Arc therapy (VMAT), Radiosurgery,
CyberKnife, and advances in Brachytherapy. These newer methods help in precisely
administering radiation dose to patients as decided by the treatment planning
system (TPS), a computer system that takes input from patient CT, medical
physicist, oncologist, medical dosimetrist and physician.
The radio-therapy treatment system depends on 3D dosimetry for pre-treatment
quality assurance. The polymer gel dosimeters are used for estimating the 3D dose
distribution using a treatment plan decided by the radiation treatment plan (RTS)
before the patient undergoes radiation exposure. Gel phantoms are prepared using
monomers to be tissue equivalent radiologically. The optical computed tomography
has been used to scan the gel dosimeters. It was observed that upon irradiation, the
monomers in gel get polymerized. Calibration measurements with varying levels of
radiation exposure show that, optical density and refractive index increase with
radiation dose. The optical density increased from (0.01 to 0.06) mm1 and the
refractive index increased from (1.34 to 1.37) for gel irradiated from (0.5 to 25) Gy
dose. The SEM imaging of calibration gels show that the particle size increases from
20nm to 400nm on radiation exposure.
The exposure of radiation to tissue causes an increase in refractive index,
thereby bending the light traversing through the tissue, resulting in deterioration in
image quality. The solution for this is to immerse the dosimeter in a refractive index
matching liquid. However, an exact match is seldom achieved. The refraction of
light passing through a dose region results in artefacts in the reconstructed images.
These refraction errors are dependent on the scanning geometry and collection
optics. The refraction arises primarily due to (1) the refractive index mismatch
between the surrounding medium and the dosimeter which results in distortions of
dose regions and (2) the refractive index changes caused by radiation dose in the
dosimeter itself that result in streaking, and quantitative errors. In order to account
for these effects and correct the distortions we used ray path modelling of light
traversed through the dosimeter. Exact path length of the ray in a discretized grid
was obtained by using ray tracing methods.
Rayline errors perturb the system when rays confront a radiation induced RI
gradient region. This is more signi ficant in 3D as the ray get deviated and does not
reach the detector plane. We extended this study to 3D, used a prototype
cone-beam scanning system to collect the projection images. We developed a fully
3D image reconstruction algorithm, algebraic reconstruction technique-refraction
correction (ART-rc) that corrects for the refractive index mismatches present in a
gel dosimeter scanner not only at the boundary, but also for any rayline refraction
due to multiple dose regions inside the dosimeter. In this study, simulation and
experimental studies have been carried out to reconstruct a 3D dose volume using
2D CCD measurements taken for various views.
Radiation dose absorbed at a tissue voxel can be calculated from kernels which
incorporate the effects of all the interactions with matter using Monte Carlo based
techniques. We studied pencil beam and point kernel based methods. Radiological
depth calculation using ray tracing technique was used for path length calculations in
a inhomogeneous phantom/patient volume. This is integrated with collapsed-cone
convolution superposition algorithm to arrive at the complete dose-distribution.
Dose reconstruction results using Monte Carlo and collapsed cone methods are
presented. The EPID image was corrected with scatter factor measurements. The
corrections improved the dose quanti cation from 88.9% to 96.5%. The resulting
dose Monitor Unit (MU) values matches well with that from TPS computation. Per
eld EPID
uence, calculated from segment wise portal images acquired using step
and shoot technique of the IMRT prostate eld is validated.
The main ndings of this study are:
1 We have demonstrated that gel dosimeters can be used to verify dose pro les
delivered using Co-60 telecobalt machines, linear accelerators, IMRT, VMAT
and Brachytherapy.
2 Refraction e ects deteriorate dose readout and induce errors in quantifying
dose. These can be overcome by using ray tracing method that calculate exact
pathlength accounting for refraction.
3 Boundary mismatch can be overcome by using exact matching liquid, but
interior refractive index changes induced by radiation can be accounted for
using our ray modelling scheme.
4 The Monte Carlo modelling of polarized light propagation in a multi-layered
turbid medium is extended to include multiparticle distribution of scatterers
and also with embedded absorbing/ scattering inhomogeneities.
5 Fluence measurements acquired using EPID along with appropriate scatter
factor corrections were found to match with those calculated by treatment
planning system (TPS). In conjunction with collapsed cone
convolution/superposition method, it can be used to compute 3D dose
distributions.
3D Image Reconstruction Using Optical Phase Retrieval And Cone-Beam TomographyHemanth, Thttps://etd.iisc.ac.in/handle/2005/15472020-10-09T06:10:38Z2011-11-22T00:00:00Z3D Image Reconstruction Using Optical Phase Retrieval And Cone-Beam Tomography
Hemanth, T
2011-11-22T00:00:00ZActive matter: chirality, translational order, and interfacesKole, Swapnilhttps://etd.iisc.ac.in/handle/2005/62002023-08-28T21:32:19ZActive matter: chirality, translational order, and interfaces
Kole, Swapnil
My PhD work is on chiral active matter with solid and fluid directions, dynamics of the
interface of a nonconserved chiral order parameter in an active system, flocking on curved
manifolds and field-driven colloids in confined nematics.
We start by formulating theories of layered active chiral matter. We
start with constructing the active model H* in two and three dimensions – the chiral
and active variant of model H. This theory describes the coupled dynamics of a conserved
scalar order parameter and a conserved momentum density field.
At thermal equilibrium, chiral molecules form a range of liquid-crystalline phases such
as cholesteric, with a helical structure of the molecular orientation. It turns that at long length scales, the mechanics of a cholesteric is precisely the same as that of a smectic
A which has an achiral one-dimensional density modulation. It is curious that
though microscopic chirality leads to a one-dimensional periodic structure, its asymptotic
long-wavelength elasticity and hydrodynamics show no signature of chirality. In this
chapter we show that this equivalence does not carry over to active cholesteric and
smectic A phases. Thanks to the presence of a mix of solid- and liquid-like
directions, we predict that chiral active stresses create a force density tangent to contours of constant mean curvature of the layers. This non-dissipative force in a fluid direction
– odder than odd elasticity – leads, in the presence of an undulational instability
created by non-chiral active stresses normal to the layers, to spontaneous vortical flows
arranged in a two-dimensional array with vorticity aligned along the pitch axis and
alternating in sign in the plane. This vortex-lattice state can be switched on or off by
means of an externally imposed uniaxial stress. We also show that a two-dimensional
active cholesteric is unstable with an activity threshold that goes to zero for an infinite
system.
We then move on to formulating the active hydrodynamics of columnar phases,
those with two solid and one fluid direction. We show that a bulk active columnar
phase is spontaneously unstable to an extensile activity along the column direction via a
buckling instability. We predict singular stiffening or softening – depending on whether
the active achiral stress is contractile or extensile – of the buckling of fluid columns in all
active columnar materials, irrespective of whether they are chiral or polar. Further, we
demonstrate that the effect of the active achiral stress in columnars is exactly equivalent
to an externally imposed in-plane, isotropic stress; therefore, the instability induced by
a singular softening of the column buckling mode – in extensile systems – is exactly
equivalent to a columnar Helfrich-Hurault instability under an external stress. This
allows us to exactly calculate the threshold activity for this instability in a finite columnar
liquid crystal. The instability is mediated by a twist-bend mode resulting in helical
columns – same as those that arise from a Helfrich-Hurault instability of passive columnar
material. If the active units composing the columnar state are in addition chiral, the
buckled and twisted state beyond the spontaneous Helfrich-Hurault instability in an
apolar system hosts large-scale shear flows due to a new form of odd elasticity. For
polar and chiral columnar systems, we show that two-dimensional solid odd elasticity is
naturally realised in this three-dimensional material. The interplay of this odd elasticity
with viscous, Stokesian hydrodynamics leads to an optical mode with a frequency that depends on the direction of the relative angle between the wavevector direction and
polarity but, crucially, not on the wavenumber. The frequency of this vibrational mode
is set by the ratio of the coefficient of chiral and polar active stress and the viscosity.
The damping of this mode is also wavenumber-independent. The oscillation is due to
the two in-plane displacement fields acting effectively as a position-momentum pair.
In chapter 4, we move to investigating the interfacial dynamics in the bulk chiral active models. In particular, we derive the stochastic partial differential equation
(SPDE) describing the dynamics of a fluctuating chiral interface with up-down symmetry
in one-dimension. The obtained SPDE has been studied before, and the
result on logarithmic corrections to scaling is stated in the Comment by Paczuski et al. However, the derivation from a bulk field theory is new, and the renormalisation
group calculation is unavailable in the literature. We derive the SPDE from an active field
theory for a non-conserved pseudoscalar field in a uniaxial medium equation, governing
the interfacial dynamics separating regions of opposite chirality. This dynamics turns
out to be equivalent to that of a steadily forced polymer. The steady-state probability
distribution of the one-dimensional shape of the domain wall is the same as the passive
Edwards-Wilkinson model. However, surprisingly, the dynamical behaviour of the
domain wall shape reveals its activity. A nonlinearity – which by scaling arguments is
marginal in one dimension – turns out to lead to anomalous growth. We examine this
numerically and analytically using a two-loop RG calculation to obtain the exponent
of the logarithmic correction to diffusivity.
The chapter 5 is on analytical modelling of experiments on AC field-driven Janus colloids in confined nematics. Electrokinetics involves study of electrically driven fluid
flow (electro-osmosis) and particle motion (electrophoresis). The use of electric fields to transport tiny particles through fluids, is an important technology
for macro-molecular sorting, colloidal assembly and a challenging area of soft-matter
research. Traditional studies on Electro-osmosis have been on colloids sus-
pended in isotropic electrolytes. Janus colloids in an isotropic electrolyte – with
dielectric and conducting hemispheres – show unidirectional motility (dielectric forward)
– thanks to the contrasting polarisability on the either hemispheres. However, this
phenomenon is unsuitable for self-assembly and micro-botic applications for its unidirec-
tional motion and eventual sedimentation in an isotropic electrolyte. In a striking depar-
ture from conventional electrophoresis, the experiments show that metal-dielectric
Janus particles can be piloted at will through a nematic liquid crystal film, in the plane
spanned by the axes of the particle and the nematic, and perpendicular to an imposed
AC electric field. A complete command over particle trajectories can be achieved by
varying field amplitude and frequency, exploiting the sensitivity of electro-osmotic flow
to the asymmetries of particle and defect structure. To understand the multi-directional
motility of janus particles in the experiments, we calculate the dipolar force density pro-
duced by the interplay of the electric field with director anchoring and the contrasting
electrostatic boundary conditions on the two hemispheres of the janus colloid to account
for the dielectric-forward (metal-forward) motion of the colloids due to induced puller
(pusher) force dipoles.
In the final chapter, we study Toner-Tu flocking on curved substrates. We
study the dynamics of density and polarisation fields on toroidal substrates and analytically calculate the steady-state profile. The Euler characteristic of the torus is zero and
hence defect-free states can exist which are well defined globally. We observe the density
profile to be inhomogeneous due to the presence of curvature. We also find delocalisa-
tion of extremas of density and polarity field. Further, the active flow allows the system
to have long-wavelength propagating sound modes which are gapped by the curvature,
while the gapless modes get localized to two special geodesics located on the positively and negatively curved faces.