5/23/05IMA Elasticity and Dynamics of LC Elastomers Leo Radzihovsky Xiangjing Xing Ranjan Mukhopadhyay Olaf Stenull

5/23/05IMA Outline Review of Elasticity of Nematic Elastomers –Soft and Semi-Soft Strain-only theories –Coupling to the director Phenomenological Dynamics –Hydrodynamic –Non-hydrodynamic Phenomenological Dynamics of NE –Soft hydrodynamic –Semi-soft with non-hydro modes

5/23/05IMA Strain Cauchy DeformationTensor (A “tangent plane” vector) Displacement strain Invariances Displacements a,b = Ref. Space i,j = Target space TCL, Mukhopadhyay, Radzihovsky, Xing, Phys. Rev. E 66, /1-22(2002)

5/23/05IMA Isotropic and Uniaxial Solid Isotropic: free energy density f has two harmonic elastic constants Uniaxial: five harmonic elastic constants Nematic elastomer: uniaxial. Is this enough?

5/23/05IMA Nonlinear strain Green – Saint Venant strain tensor- Physicists’ favorite – invariant under U; u is a tensor is the reference space, and a scalar in the target space

5/23/05IMA Spontaneous Symmetry Breaking Phase transition to anisotropic state as m goes to zero Direction of n 0 is arbitrary Symmetric-Tracelesspart Golubovic, L., and Lubensky, T.C.,, PRL 63, , (1989).

5/23/05IMA Strain of New Phase d u is the deviation of the strain relative to the original reference frame R from u 0 u’ is the strain relative to the new state at points x’ d u is linearly proportional to u’

5/23/05IMA Elasticity of New Phase Rotation of anisotropy direction costs no energy C 5 =0 because of rotational invariance This 2nd order expansion is invariant under all U but only infinitesimal V

5/23/05IMA Soft Extensional Elasticity Strain u xx can be converted to a zero energy rotation by developing strains u zz and u xz until u xx =(r-1)/2

5/23/05IMA Frozen anisotropy: Semi-soft System is now uniaxial – why not simply use uniaxial elastic energy? This predicts linear stress-stain curve and misses lowering of energy by reorientation: Model Uniaxial system: Produces harmonic uniaxial energy for small strain but has nonlinear terms – reduces to isotropic when h=0 f (u) : isotropic Rotation

5/23/05IMA Semi-soft stress-strain Ward Identity Second Piola-Kirchoff stress tensor; not the same as the familiar Cauchy stress tensor Ranjan Mukhopadhyay and TCL: in preparation

5/23/05IMA Semi-soft Extensions Not perfectly soft because of residual anisotropy arising from crosslinking in the the nematic phase - semi-soft. length of plateau depends on magnitude of spontaneous anisotropy r. Warner-Terentjev Stripes form in real systems: semi-soft, BC Break rotational symmetry Finkelmann, et al., J. Phys. II 7, 1059 (1997); Warner, J. Mech. Phys. Solids 47, 1355 (1999) Note: Semi-softness only visible in nonlinear properties

5/23/05IMA Soft Biaxial SmA and SmC Free energy density for a uniaxial solid (SmA with locked layers) C 4 =0: Transition to Biaxial Smectic with soft in-plane elasticity C 5 =0: Transition to SmC with a complicated soft elasticity Red: Corrections for transition to biaxial SmA Green: Corrections for trtansition to SmC Olaf Stenull, TCL, PRL 94, (2005)

5/23/05IMA Coupling to Nematic Order Strain u ab transforms like a tensor in the ref. space but as a scalar in the target space. The director n i and the nematic order parameter Q ij transform as scalars in the ref. space but, respectively, as a vector and a tensor in the target space. How can they be coupled? – Transform between spaces using the Polar Decomposition Theorem. Ref->targetTarget->ref

5/23/05IMA Strain and Rotation Simple Shear Symmetric shear Rotation

5/23/05IMA Softness with Director Director relaxes to zero N a = unit vector along uniaxial direction in reference space; layer normal in a locked SmA phase Red: SmA-SmC transition

5/23/05IMA Harmonic Free energy with Frank part

5/23/05IMA NE: Relaxed elastic energy Uniaxial solid when C 5 R >0, including Frank director energy

5/23/05IMA Slow Dynamics – General Approach Identify slow variables :  Determine static thermodynamics: F(  ) Develop dynamics: Poisson-brackets plus dissipation Mode Counting (Martin,Pershan, Parodi 72): –One hydrodynamic mode for each conserved or broken-symmetry variable –Extra Modes for slow non-hydrodynamic –Friction and constraints may reduce number of hydrodynamics variables

5/23/05IMA Preliminaries Harmonic Oscillator: seeds of complete formalism friction Poisson bracket Poisson brackets: mechanical coupling between variables – time-reversal invariant. Dissipative couplings: not time-reversal invariant Dissipative: time derivative of field (p) to its conjugate field (v)

5/23/05IMA Fluid Flow – Navier Stokes Conserved densities: mass : r Energy: e Momentum: g i = rv i

5/23/05IMA Crystalline Solid I Conserved densities: mass : rEnergy: e Momentum: g i = rv i Broken-symmetry field : Phase of mass-density field: u describes displacement of periodic part of density Mass density is periodic Strain Free energy Aside: Nonlinear strain is not the Green Saint-Venant tensor

5/23/05IMA Crystalline Solid II permeation Modes: Transverse phonon: 4 Long. Phonon: 2 Permeation (vacancy diffusion): 1 Thermal Diffusion: 1 Permeation: independent motion of mass-density wave and mass: mass motion with static density wave Aside: full nonlinear theory requires more care

5/23/05IMA Tethered Solid Isotropic elastic free energy 7 hydrodynamic variables: 1 density,3 momenta, 3 displacements, 1 energy + 1 constraint = 8-1=7 Classic equations of motion for a Lagrangian solid; use Cauchy-Saint-Venant Strain + energy mode (1)

5/23/05IMA Gel: Tethered Solid in a Fluid Tethered solid Fluid Frictional Coupling Total momentum conserved Fast non-hydro mode: but not valid if there are time scales in G Fluid and Solid move together Friction only for relative motion- Galilean invariance

5/23/05IMA Nematic Hydrodynamics: Harvard I g is the total momentum density: determines angular momentum Frank free energy for a nematic

5/23/05IMA Nematic Hydrodynamics: Harvard II w – fluid vorticity not spin frequency of rods Symmetric strain rate rotates n permeation Stress tensor can be made symmetric Modes: 2 long sound, 2 “slow” director diffusion. 2 “fast” velocity diff.

5/23/05IMA NE: Director-displacement dynamics Director relaxes in a microscopic time to the local shear – nonhydrodynamic mode Stenull, TCL, PRE 65, (2004) Tethered anisotropic solid plus nematic Semi-soft: Hydrodynamic modes same as a uniaxial solid: 3 pairs of sound modes Note: all variables in target space Modifications if g depends on frequency

5/23/05IMA Soft Elastomer Hydrodynamics Same mode structure as a discotic liquid crystal: 2 “longitudinal” sound, 2 columnar modes with zero velocity along n, 2 smectic modes with zero velocity along both symmetry directions Slow and fast diffusive modes along symmetry directions

5/23/05IMA Beyond Hydrodynamics: ‘Rouse’ Modes Standard hydrodynamics for wt E >1

5/23/05IMA Rouse in NEs References: Martinoty, Pleiner, et al.; Stenull & TL; Warner & Terentjev, EPJ 14, (2005) Second plateau in G' “Rouse” Behavior before plateau

5/23/05IMA Rheology Conclusion: Linear rheology is not a good probe of semi- softness

5/23/05IMA Summary and Prospectives Ideal nematic elastomers can exhibit soft elasticity. Semi-soft elasticity is manifested in nonlinear phenomena. Linearized hydrodynamics of soft NE is same as that of columnar phase, that of a semi-soft NE is the same as that of a uniaxial solid. At high frequencies, NE’s will exhibit polymer modes; semisoft can exhibit plateaus for appropriate relaxation times. Randomness will affect analysis: random transverse stress, random elastic constants will complicate damping and high-frequency behavior.