The Curious Case of Soft Matter Ranjini Bandyopadhyay Raman Research Institute

Transcription

1 The Curious Case of Soft Matter Ranjini Bandyopadhyay Raman Research Institute My research interests: structure, dynamics, phase behavior and flow of soft materials Panta rei: everything flows (Heraclitus circa 500 BC) Given enough time even mountains will flow! (The mountains flowed before the lord: Deborah, prophetess of the God of the Israelites) Soft materials are those materials that show solid-like and liquid-like behaviors at time scales that are accessible in the laboratory: few seconds to few hours. They can go from solid to liquid under the imposition of rather small forces.

2 Solids and liquids 1. Rigid 2. Does not flow 3. Retains its shape and volume 4. Atoms packed very tightly together 5. Stores energy Hookes law s = Eg 1. Flows 2. Takes the shape of the container 3. Retains its volume but not shape 4. Dissipates (loses) energy Newton s law of viscosity s = hg

3 Examples of soft materials Polymer chain (Plastics) Sand Foam toothpaste Paint Clay Liquid crystal

4 What is Soft Matter? What do we mean by soft matter? Americans prefer to call it complex fluids. This is a rather ugly name which tends to discourage the young students. But it does indeed bring in two of the major features: complexity and flexibility. P.-G de Gennes, Nobel Lecture, December 9, 1991 Pierre de Gennes, Orsay, FRANCE, ( ). Complexity Flexibility (a story of boots) Paramecium wait for a few billion years Us! Rubber tree Rubber Tyres (Charles Goodyear) Rubber boots (Native American)

5 Those in-between squishy materials and the science of viscoelasticity (non-newtonian flow) Blood, polymer, paint, foam, food lie in between ideal the ideal elastic solid and the ideal viscous fluid Paint is thixotropic: it shear thins and then thickens immediately after the paint brush is withdrawn. Silly Putty/ slime: G is a function of time Marmalade, foam: h is a fn. of rate of deformation

6 Why do we study soft materials in the laboratory? Big and slow! You can study them with a microscope, by autocorrelating scattered light intensity and even with a commercial digital camera Can be pushed very easily! Go from solid-like to liquid-like on the application of very small applied forces. You can study them by applying forces easily accessible in the laboratory Distinctive physics: interesting at several length scales. Excellent model systems to understand materials hard to do experiments with. Very important technological, pharmaceutical applications. Eg: glasses and convection, plastics and drug delivery etc.

7 Soft materials we use in the lab Foam: gas bubbles suspended in a liquid; interesting because of their topology and also because of their highly nonequilibrium nature: time evolution of structure arising out of coarsening, drainage and bubble collapse eg. Gillette Foamy. Flow: stick-slip Polymer: a macromolecule with repeating structural units; at high enough concentrations they form knots which can break spontaneously due to thermal fluctuations or upon shearing eg. Rubber, POLYOX, PDMS Micelles: aggregates of diverse structures formed from amphiphilic molecules eg. detergents Colloidal suspensions: consist of a dispersed phase (micron-sized globules of polymers) dispersed within a continuous phase (water). The dispersed phase stays afloat because of Brownian motion and the interaction mechanisms are excluded volume interaction, van der Waals interaction, screened repulsion, entropic forces, steric repulsions eg. clay suspensions

8 Glass (both window glass and colloidal soft glasses) Crystal: atoms are stacked periodically (like cannonballs) Glass: atoms are packed irregularly and tightly (underlying structure is liquid-like (NOT solid-like). Characterized by slow dynamics and aging A metastable system How do the molecules move? Because of thermal fluctuations, particles are ALWAYS in motion Two kinds of motion: vibration within the cage ( relaxation) occasional hopping out of the cage ( relaxation) You can make a soft glass using colloidal particles which are bigger (and easier to observe) and interact less strongly (so it is easier to make it flow) A question of time scales relaxation relaxation Window glass seconds 1000 years Soft Clay Glass 1 second seconds

9 Persistent holes and cornstarch monsters in vibrated cornstarch! (demonstrated by Prakhyat and Rajib at RRI) Cornstarch in water 35% w/w vibrated on a shaker (sinusoidal vibrations 1 mm amplitude and 110 Hz frequency) First we see ripples, then we see persistent holes and eventually cornstarch monsters

10 Cornstarch dilates or shear-thickens (when you pull or push or shake or stir it hard, it becomes thicker!) When you run fast on a pool of cornstarch, it behaves like a solid, BUT When you stand on the same pool, you sink! ( An experiment with shampoo or liquid hand soap The Kaye effect: when shampoo/ soap is poured on a slanted surface, a heap forms, And occasionally a secondary jet emerges from the heap in random directions. This is also seen in paints and is due to shear thinning: the down-going stream "slips" off the pile it is forming, and due to a thin layer of shear-thinned liquid acting as a lubricant, it does not combine with the pile. When this stream encounters a dimple in the pile, it will shoot up, giving rise to this effect.

11 Experiment with some polymers - I The Weissenberg effect: rod climbing effect in stirred polymer solutions In contrast, if water is stirred, the surface is concave due to inertial forces (the water surface is highest at the sides and at the lowest at the center) Experiment with some polymers - II The Barus effect: a polymer solution forced out through the hole of a tube, swells a lot spontaneously! In contrast, water maintains the same radius Experiment with some polymers - III Fano flow: a polymer solution or egg white can be drawn out with a syringe (siphoned out) even when the syringe/ syphon is way above the surface of the polymer solution/ egg white!

12 Sand: another odd material it can be solid, liquid and gas depending on how much you push/pull it! A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (for example, through friction when grains collide) The rate of flow of sand does not depend on the height of sand above! This is because the weight of sand is supported not just by the base, but also by the sides. In contrast, for water, the taller the column of water, the faster it would have flowed out had there been a hole at the bottom Some other materials that behave similarly (nuts, cornflakes, rice, pills etc.) They are called granular matter

13 Size segregation in vibrated granular matter From sixtysymbols.com Fig (I): the Brazil nut effect: the bigger nut comes out on top When you shake the container up an down! Fig (II) : orange PS beads (1.5 mm diameter, 200kg/m 3 ) atop blue silica pebbles (2.8 mm diameter, 700 kg/m 3 ). (I) (II)