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ENTROPY IN SOFT MATTER PHYSICS
Author: Tim Verbovšek
Mentor: doc. dr. Primož Ziherl
Entropy in soft matter physics
Overview
Entropy Polymers Depletion potential
Experiment Liquid crystals
Simulation
Entropy in soft matter physics
Entropy
2nd Law of thermodynamics In equilibrium, the system has maximal entropy Written in mathematical form by Rudolf Clausius
Free energy
Hard-core interactions
Entropy in soft matter physics
In Statistical Physics
Macrostate: property of the system Microstate: state of a subunit of the system Ω statistical weight
Different sets of microstates for a given macrostate if all sets of microstates are equally probable
Entropy in soft matter physics
In Statistical Physics
Entropy in soft matter physics
Polymers
Long chains Random walk Real polymer
chains Entropic spring
Entropy in soft matter physics
Ideal Polymer Chains
Random walk Persistence length
Approximate length at which the polymer loses rigidity
Gaussian probability distribution of the end-to-end vector size exp()
Configurational entropy:
Free energy:
Entropy in soft matter physics
Ideal Polymer Chain
Entropy in soft matter physics
Real Polymer Chains
Correlation of neighbouring bonds Finite bond angle
Excluded volume Self-avoiding walk; the polymer
cannot intersect itself The coil takes up more space
Entropy in soft matter physics
Depletion Potential
Macrospheres and microspheres
Exclusion zone Asakura-Oosawa
model (1954) The result of
overlapping exclusion zones is an attractive force between macrospheres
Microscopic image of milk. Droplets of fat can be seen.
Entropy in soft matter physics
Depletion Zone
An excluded zone appears around the plate submerged in a solution of microspheres
Entropy in soft matter physics
Depletion Zone
Exclusion zones overlap, leading to a larger available volume for the microspheres
Entropy in soft matter physics
Depletion Potential
Ideal gas of microspheres Free energy is
Entropic force: Two spheres:
) Wall-sphere:
Short ranged interactions
Entropy in soft matter physics
Measuring the Forces
Silica beads were suspended in a solution of λ-DNA polymers
Measurement of the positions of the beads gives the probability distribution P(r)
Entropy in soft matter physics
Measuring the Forces
Optical tweezers hold the beads in place
The potential as a result of optical tweezers was found to be parabolic
Entropy in soft matter physics
Measuring the Forces
Entropy in soft matter physics
Measuring the Forces
Experiment gives a good fit to the Asakura-Oosawa model
The range of the depletion potential was found to be
Depth of the potential increases linearly with polymer concentration
)
Entropy in soft matter physics
Liquid Crystals
Isotropic phase Nematic phase
Director Positions of the centers of mass are
isotropic Smectic phase
Layers Smectic A Smectic C
Columnar Disk-shaped molecules
Entropy in soft matter physics
Phase Transitions
Onsager theory (1949) Solid rod model
- orientational entropy Has a maximum in the isotropic phase
- packing entropy It is maximised when the molecules are parallel The same role as the depletion potential in colloidal
dispersions It is a linear function of the concentration of rods
Entropy in soft matter physics
The Simulation
Lyotropic liquid crystals: Phase changes occur by changing the molecule concentration (T = const.)
Computer simulations for hard spherocylinders Shape anisotropy parameter Length-to-width ration
Entropy in soft matter physics
The Results
Entropy in soft matter physics
Summary
Entropy With hard spheres and constant temperature, the free
energy depends only on entropy Polymers
Entropic spring Depletion potential
Short-range attraction between colloids Experiment
Liquid crystals Phase transitions Simulation