Roberto Piazza
Department of Chemistry, Materials Science
and Chemical Engineering “Giulio Natta”
Politecnico di Milano
Soft Matter in external fields and in non-equilibrium conditions
Aim and overall plan of the course
Soft materials have countless applications in all primary and manufacturing industrial sectors, in the consumer market,
in the development of new technologies. This is extremely useful and nice, but is not what this course is about.
Rather, I maintain that the investigation of soft matter, particularly when performed in the presence of external fields
or in non-equilibrium conditions, is blessed by another, innermost fascination: the power to address and solve a large
number of basic problems in statistical and condensed matter physics. This claim will hopefully be substantiated by
focusing on three specific subjects:
i) The equilibrium and kinetic behavior of soft matter in the presence of gravity, arguably the simplest and surely the
most pervading external field (L1, L3);
ii) The motion of colloidal particles driven by the elusive forces due to the presence of a thermal gradient, a prototypal
non-equilibrium condition (L4);
iii) The novel and challenging effects stemming from providing an irreversible energy supply to thermal agitation (L5).
An “intermezzo” lecture (L2) will be specifically dedicated to those novel experimental techniques which may prove to
be particularly useful to address all the former subjects. A provisional outline of the single lectures is given in the
following.
L1 (May 7): Settled, unsettled, and unsettling issues in particle settling
Why complex fluids may in fact be simple: colloids as model systems for statistical mechanics. The unbearable heaviness of colloids:
gravity settling. Sedimentation: a privileged route to the equation of state and phase behaviour. When entropy rules: hard & sticky
spheres. Electric thrills: settling of charged colloids. Give it a shove! The largely uncharted world of centrifugal settling. Akin to
Janus: the quiet and frantic faces of settling kinetics. When stuffing means promoting: settling of strongly attractive colloids. A final
surprise: buoyancy in a crowded world.
L2 (May 13): When the going gets tough, the tough gets going: novel methods for novel challenges
A speckled world: the subtle role of spatial coherence in optics. What an old radio can teach us: the very nature of Dynamic Light
Scattering. Short-sighted DLS. Microscope: not just a powerful magnifying lens. Blending Scattering with Imaging: Photon
Correlation Imaging and Differential Dynamic Microscopy. The Digital Fourier Macroscope. Tracing without tracers: Ghost Particle
Velocimetry.
L3 (May 21): Further gravity tricks: restructuring and ageing of colloid and polymer gels
Revisiting irreversible colloidal aggregation under a “new light” (literally). The fascinating world of crushing, squeezing, and crystalbudding depletion gels. Sudden rainstorms in apparently frozen structures. Inconstant solids: gelation, restructuring, and creeping
of biopolymer gels with physical cross-links. Frenzy to merge: clues about the calamitous behavior of amyloid proteins.
L4 (May 28): Thermophoresis and the treacherous world of non-equilibrium
A little story about light mills, referees, and stress: thermophoresis in gases. Discrimination in transport: what is phoretic motion,
and common misconceptions about it. Phantom fields and thermal forces: colloid thermophoresis. Temperature as a novel,
selective tool to split up and draw apart particles and macromolecules. From Navier & Stokes plausible explanation to Langevin’s
puzzlement: why does a Brownian particle really moves in a thermal gradient? Thermophoresis as an emblematic non-equilibrium
effect: the evasive nature of Non-Equilibrium Stationary States.
L5 (June 4): Supplying fuel to thermal agitation: active and vital particles
Moving by internal forces in the Aristotelian world: general properties of active motion and of the hydrodynamic interactions they
brings about. Gravity strikes back: sedimentation of active colloids. Concentrated suspensions of restless particles: just a question
of effective temperature? Running and tumbling: bacteria-driven ratchets, rectification, and the arrow of time. Whirls and eddies in
bacteria crowds: turbulence with an inverse energy cascade. Why does active motion abate viscosity? Flowing bacteria, Reynolds
stresses, and intermittency.