Introduction to Soft Matter PH3-SM (PHY3032) Soft Matter Physics 4 October, 2010 Lecture 1: Introduction to Soft Matter

What is Condensed Matter? “Condensed matter” refers to matter that is not in the gas phase but is condensed as a liquid or solid. (condensed denser!) Matter condenses when attractive intermolecular bond energies are comparable to or greater than thermal (i.e. kinetic) energy ~ kT. Phase diagram of carbon dioxide Image: http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_10.html

Phase Diagram of Water Image: http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_10.html

Soft (Condensed) Matter Refers to condensed matter that exhibits characteristics of both solids and liquids. The phrase “soft matter” was used by Pierre de Gennes as the title of his 1991 Nobel Prize acceptance speech. Soft matter can flow like liquids ( has a measurable viscosity). Soft matter can bear stress and recover its original shape after deformation (i.e. is elastic). Viscoelastic behaviour = viscous + elastic Examples: rubbers, gels, pastes, creams, paints, soaps, liquid crystals, proteins, cells, tissue, humans?

Types of Soft Matter: (1) Colloids A colloid consists of sub-mm particles (but not single molecules) of one phase dispersed in a continuous phase. The size scale of the dispersed phase is between 1 nm and 1 mm. The dispersed phase and the continuous phases can consist of either a solid (S), liquid (L), or gas (G): Dispersed Phase Continuous Name Examples L/S G aerosol fog, hair spray; smoke G L/S foam beer froth; shaving foam; poly(urethane) foam L L (S) emulsion mayonnaise; salad dressing S L sol latex paint; tooth paste S S solid suspension pearl; mineral rocks There is no “gas-in-gas” colloid, because there is no interfacial tension between gases!

Interfacial Area of Colloids For a spherical particle, the ratio of surface area (A) to volume (V) is: r Thus, with smaller particles, the interface becomes more significant. A greater fraction of molecules is near the surface. Consider a 1 cm3 phase dispersed in a continuous medium: No. particles “Particle” volume(m3) Edge length (m) Total surface area(m2) 1 10-6 10-2 0.0006 103 10-9 10-3 0.006 106 10-12 10-4 0.06 109 10-15 10-5 0.6 1012 10-18 10-6 6.0 1015 10-21 10-7 60 1018 10-24 10-8 600

Colloidal Flow Properties Shear thickening behaviour of a polymer colloid (200 nm particles of polymers dispersed in water): At a low shear rate: flows like a liquid At a high shear rate: solid-like behaviour

Types of Soft Matter: (2) Polymers A polymer is a large molecule, typically with 50 or more repeat units. (A “unit” is a chemical group.) Examples include everyday plastics (polystyrene, polyethylene); rubbers (also called elastomers); biomolecules, such as proteins and starch. Physicist’s view of a polymer: Each “pearl” on the string represents a “repeat unit” of several atoms, linked together by strong covalent bonds. For instance, in a protein molecule, the repeat units are amino acids. Starch consists of repeat units of sugar. The composition of the “pearls” is not important (for a physicist!). Physics can predict the size and shape of the molecule; the important parameter is the number of repeat units, N.

Terminology of Polymers A “plastic” is a more solid-like polymer. When it is deformed beyond a certain limit, the deformation becomes permanent, and it is called plastic deformation. When polymers are at higher temperatures, the molecules move with greater mobility, and flow is possible. When polymer chains are “tied together” by chemical bonds, the polymer remains deformable, but it obtains elastic properties. When stress is released, the material recovers its initial size and shape. This type of polymer is a called a rubber or an elastomer. Polymers can be dissolved in a liquid (called a solvent) to make a solution. Elastic Stress Plastic Chain network in an elastomer. Strain

Types of Soft Matter: (3) Liquid Crystals • A liquid crystal is made up of molecules that exhibit a level of ordering that is intermediate between liquids (randomly arranged and oriented) and crystals (three-dimensional array). Flows easily in the aligned direction. Elastic in the normal direction. Image: http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_10.html This form of soft matter is interesting and useful because of its anisotropic optical and mechanical properties.

Monodisperse Particle Size Characteristics of Soft Matter (4 in total) (1) Relevant length scales between the atomic and the macroscopic Top view 3 mm x 3 mm scan Edge length = 1 mm Vertical scale = 200 nm Acrylic Latex Paint Monodisperse Particle Size Example of colloidal particles

Typical Length Scales Atomic spacing: ~ 0.1 nm “Pitch” of a DNA molecule: 3.4 nm Diameter of a surfactant micelle: ~6-7 nm Radius of a polymer “chain” molecule: ~10 nm Diam. of a colloidal particle (e.g. in emulsion paint): ~200 nm Bacteria cell: ~2 mm Diameter of a human hair: ~80 mm

Poly(ethylene) crystal Typical Length Scales Poly(ethylene) crystal Crystals of poly(ethylene oxide) 15 mm x 15 mm 5 mm x 5 mm Polymer crystals can grow up to millimeters in size!

Intermediate Length Scales Mathematical descriptions of soft matter can ignore the atomic level. “Mean field” approaches define an average energy or force imposed by the neighbouring molecules. Physicists usually ignore the detailed chemical make-up of molecules; can treat molecules as “strings”, rods or discs.

Characteristics of Soft Matter (2) Weak short-range forces and interfaces are important. From Materials World (2003) The structure of a gecko’s foot has been mimicked to create an adhesive. But the attractive adhesive forces can cause the synthetic “hairs” to stick together.

Chemical Bonds in Soft Matter • In “hard” condensed matter, such as Si or Cu, strong covalent or metallic bonds give a crystal strength and a high cohesive energy (i.e. the energy to separate atoms). • In soft matter, weaker bonds - such as van der Waals - are important. Bond energy is on the same order of magnitude as thermal energy ~ kT. (k is Boltzmann’s constant: 1.38 x 10-23 J/K) • Hence, bonds are easily broken and re-formed. • The strength of molecular interactions (e.g. charge attractions) decays with distance, r, between molecules or particles. • At distances less than 10 nm, they start to become significant. r

Condensed Matter and the Origin of Surface Tension Increasing density Meniscus From I.W. Hamley, Introduction to Soft Matter Liquids and gases are separated by a meniscus; they differ only in density but not structure (i.e. arrangement of molecules in space). • Molecules at an interface have asymmetric forces around them. • In reducing the interfacial area, molecules are forced below the surface, where they are completely surrounded by neighbours. • Force associated with separating neighbouring molecules = surface tension.

Interface with air = “surface” Interfacial Energy An interfacial energy G is associated with the interface between two phases (units of Jm-2) (also called an interfacial tension: Nm-1) F d G q Interface with air = “surface” For mercury, G = 0.486 N/m For water, G = 0.072 N/m For ethanol, G = 0.022 N/m Mercury has a very high surface energy! What characteristics result from a high surface energy? Image: http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_10.html

Contact Angle: Balance of Forces Imagine a 10 mL drop of liquid on a solid. (No effect of g.) GLA SA energy is equivalent to ½ of the energy to cleave the solid liquid GSL q GSA solid Three interfaces: solid/liquid (SL); liquid/air (LA); solid/air (SA) Each interface has a tension (energy): GSL; GLA; GSA At equilibrium, lateral tensions must balance: Contact angle measurements thus provide information on surface tensions.

Hydrophobicity and Hydrophilicity water Fully wetting solid water solid q Hydrophilic q is <90 water q http://scottosmith.com/2007/10/03/water-beads/ solid Hydrophobic q is >90

Lotus Leaf Inspired Synthetic Super-hydrophobic Surfaces Laser-patterned surface Lotus leaf: low surface energy plus textured. DOI: 10.1117/2.1200901.1441 V. Zorba, et al., Biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf, Adv. Mater. (2008) 20, pp. 4049-4054. M. Barberoglou, et al., Bio-inspired water repellent surfaces produced by ultrafast laser structuring of silicon, Appl. Surf. Sci. (2009) 255, pp. 5425-5429.

Characteristics of Soft Matter (3) The importance of thermal fluctuations and Brownian motion Brownian motion can be thought of as resulting from a slight imbalance of momentum being transferred between liquid molecules and a colloidal particle.

Thermal fluctuations Vz V Vy Vx Soft condensed matter is not static but in constant motion at the level of molecules and particles. The “equipartition of energy” means that for each degree of freedom of a particle to move, there is kT/2 of thermal energy. For a colloidal particle able to undergo translation in the x, y and z directions, the thermal energy is 3/2 kT. k = 1.38 x 10-23 JK-1, so kT = 4 x 10-21 J per molecule at room temperature (300 K). kT is a useful “gauge” of bond energy. Vx Vy Vz V The kinetic energy for a particle of mass, m, is 1/2 mv2 = 3/2 kT. When m is small, v becomes significant.

Thermal motion of a nano-sized beam In atomic force microscopy, an ultra-sharp tip on the end of a silicon cantilever beam is used to probe a surface at the nano-scale. By how much is the beam deflected by thermal motion? For AFM applications, the cantilever beam typically has a spring constant, kS, of ~ 10 N/m. The potential energy required for deflection of the beam, Ed, by a distance, X is Ed = ½ kSX 2. At a temperature of 300 K, the thermal energy, E, is on the order of kT = 4 x10-21 J. This energy will cause an average deflection of the beam by X = (2E/kS)0.5  1 x 10-7 m or 100 nm. Polymers and membranes can have an even lower spring constant! 100 mm x 30 mm x 2 mm X

Characteristics of Soft Matter (4) Tendency to self-assemble into hierarchical structures (i.e. ordered on size scales larger than molecular) Image from IBM (taken from BBC website) Two “blocks” in one polymer chain Diblock copolymer molecules spontaneously form a pattern in a thin film. (If one phase is etched away, the film can be used for lithography.)

Polymer Self-Assembly Poly(styrene) and poly(methyl methacrylate) copolymer AFM image Diblock copolymer 2mm x 2mm Layers or “lamellae” form spontaneously in diblock copolymers.

Spider Silk: An Example of a Hierarchical Structure Amino acid units P. Ball, Nanotechnology (2002) 13, R15-R28

DNA Base Pairs Drive the Self-Assembly of Helices Adenine (A) complements thymine (T) with its two H bonds at a certain spacing. Guanine (G) complements cytosine (C) with its three H bonds at different spacings. Example of DNA sequence: ATCGAT TAGCTA

Designed Nanostructures from DNA Strands of DNA only bind to those that are complementary. DNA can be designed so that it spontaneously creates desired 3-D structures. N C Seeman 2003 Biochemistry 42 7259-7269

Particles Can Assemble into Colloidal Crystals MRS Bulletin, Feb 2004, p. 86 Colloidal particles can have a +ve or -ve charge. In direct analogy to salt crystals of +ve and -ve ions, charge attractions can lead to close-packing in ordered arrays.

Colloidosomes: Self-assembled colloidal particles Colloidal particles (<1 mm) Liquid B Liquid A A.D. Dinsmore et al., “Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles,” Science, 298 (2002) p. 1006.

Hydrophilically-driven self-assembly of particles I. Karakurt et al., Langmuir 22 (2006) 2415

Surfactants at Interfaces emulsion Work (W) is required to increase the interfacial area (A): “oil” water Interfacial tension, G Typical G values for interfaces with water - carbon tetrachloride: 45 mN/m; benzene: 35 mN/m; octanol: 8.5 mN/m Surfactants reduce G. Are used to make emulsions using less W and to achieve “self assembly” (i.e. spontaneous organisation) A surfactant (surface active agent) molecule has two ends: a “hydrophilic” one (attraction to water) and a “hydrophobic” (not attracted to water) one. Commonly known as soap!

Examples of Surfactant Self-Assembly water Surfactant (b) (a) Spherical end is hydrophilic. Tail is hydrophobic. (c) (d) From I.W. Hamley, Introduction to Soft Matter Surfactants can assemble into (a) spherical micelles, (b) cylindrical micelles, (c) bi-layers (membranes), or (d) saddle surfaces in bicontinuous structures depending on their concentration and the balance between their hydrophobic and hydrophilic components.

Examples of Surfactant Self-Assembly The “plumber’s nightmare” From RAL Jones, Soft Condensed Matter Surfactants can create a bi-continuous surface to separate an oil phase and a water phase. The hydrophilic end of the molecule orients itself towards the aqueous phase. The oil and water are completely separated but both are CONTINUOUS across the system.

Materials with controlled structure obtained through self-assembly Micelles are removed to leave ~ 10 nm spherical holes. Structure has low refractive index. Can be used as a membrane. Surfactant micelles (soft “nano-objects”) are packed together SiO2 (silica) is grown around the micelles P. Ball, Nanotechnology (2002) 13, R15-R28

Competitions in Self-Assembly DF = DU - TDS If a process decreases the free energy (DF < 0) of a system, then the process happens spontaneously. Entropy (S) increase is favourable Internal Energy (U) decrease is favourable Surfactant molecules segregate at an interface in order to LOWER the interfacial energy (U) - leading to an ordering of the system. This self-assembly is opposed by thermal motion that disrupts the ordering. Self-assembly usually DECREASES the entropy, which is not favoured by thermodynamics. But there are attractive and repulsive interactions between molecules (lowering U) that can dominate.

Importance of Interfaces There is thermodynamic work (W) associated with increasing or decreasing the interfacial area, A, of a substance: dW = GdA Doing work on a system will raise its internal energy (U; dU = dW + dQ)) and hence its free energy (F). An increase in area raises the system’s free energy, which is not thermodynamically favourable. So, sometimes interfacial tension opposes and destroys the formation of small phases. An example is coalescence in emulsions.

Coalescence in Emulsions Liquid droplet volume is the same before and after coalescence: r R Surface area of droplet made from coalesced droplets: 4pR2 Surface area of N particles: 4Npr2 Change in area, DA = - 4pr2(N-N2/3) In 1 L of emulsion (50% dispersed phase), with a droplet diameter of 200 nm, N is ~ 1017 particles. Then DA = -1.3 x 104 m2 With G = 3 x 10-2 J m-2, DF =GDA = - 390 J.

Nanotechnology Science Fact or Fiction? A vision of “nanorobots” travelling through the a blood vessel to make repairs (cutting and hoovering!). An engine created by down-scaling a normal engine to the atomic level http://physicsworld.com/cws/article/print/19961 K Eric Drexler/Institute for Molecular Manufacturing, www.imm.org.

Key Limitations for Nanorobots and Nanodevices (1) Low Reynolds number, Re : viscosity is dominant over inertia. (2) Brownian and thermal motion: there are no straight paths for travel and nothing is static! (Think of the AFM cantilever beam.) (3) Attractive surface forces: everything is “sticky” at the nano-scale. Is not easy to slide one surface over another. Why not make use of the length scales and self assembly of soft matter?

Viscous Limitation for “Nanorobot Travel” Reynolds’ Number: (Compares the effects of inertia (momentum) to viscous drag) a V = velocity r = density of the continuous medium h = viscosity of the continuous medium When Re is low, the viscosity dominates over inertia. There is no “coasting”!

Alternative Vision of a Nano-Device Closed state: K+ cannot pass through Open state: K+ can pass through A channel that allows potassium ions to pass through a cell membrane but excludes other ions. The nanomachine can be activated by a membrane voltage or a signalling molecule. Flexible molecular structure is not disrupted by thermal motion. http://physicsworld.com/cws/article/print/19961