On the Willmore Functional and Applications

Transcription

1 On the Willmore Functional and Applications Yann Bernard Monash University La Trobe University January 26, 2017

2 Some History ca. 1740: Leonhard Euler and Daniel Bernoulli study the 1-dim. elastica ca. 1815: Sophie Germain introduces mean curvature ca. 1925: Wilhelm Blaschke tries to tie minimal surfaces and conformal invariance in 1965: Tom Willmore (re)discovers the Willmore functional

3 A Biological Motivation Production rate: / sec. Total: = 70% of your cells No nucleus, no organelle, no DNA Size: 7µm diameter and 1µm thick. Heatlhy" red blood cells look like biconcave disks Modelled as droplet of liquid (haemoglobin) within thin plasmic membrane

4 Membrane is a lipid bilayer Membrane is incompressible (observation): area and volume constant If no shearing/stretching: which force dictates shape? Canham, Evans, Deuling, Helfrich postulate: the elastic energy.

5 Modeling the Energy Exterior heads have available: L + = (R + δ)θ Interior heads have available: L = (R δ)θ Tails have available: L = Rθ L 0 Energy modeled by elastic energy of a spring with intrinsic stiffness σ (N): E ± = σ 2L 0 (L ± L 0 ) 2 Total energy of membrane element: E = E + + E = σδ 2 L 0 R 2

6 Sum all infinitesimal contributions to obtain the energy 1 E = µ R dl, µ := 2 σδ2. membrane R at point p is the radius of best-fitting circle tangent to membrane at p.

7 For a 2-dimensional membrane: ( 1 E = µ + 1 ) 2 ds, [µ] = Joules. R 1 R 2 membrane

8 Mean curvature : H = 1 2 (κ 1 + κ 2 ) Willmore energy : W (Σ) := Σ H 2 da

9 Minimizing the Energy Fixing area A and volume V, the energy is: E(Σ) = H 2 da αa(σ) βv (Σ). Many shapes are possible (how many? what are they?) Define the reduced volume: λ := 6 π V A. 3/2 Imposing axial symmetry: Σ Red blood cell: V 100µm 3 and A 140µm 2, so λ 0.64.

10 However... Not all red blood cells are biconcave disks Model needs to be modified to incorporate various physical effects

11 Vesicles Small tenuous objects with bilayer membrane of surfactant molecules (e.g. phospholipids). Look like deformed bags (1 to 100 µm). Useful to study emulsions and micro-emulsions, mixtures used in human activity (cooking, painting, oil extraction, cosmetics, drug delivery,...) Unlike red blood cells, vesicles may display non-trivial topology.

12 Introduction to the Willmore Functional For a closed oriented surface Σ without boundary, the Willmore energy is W (Σ) = H 2 da. Willmore energy invariant under reparametrizations in domain and conformal transformations (translation, rotation, dilation, inversion) in target Σ

13 Willmore energy appears in various areas of science cell biology : bending energy of elastic membranes (biomembranes, vesicles, smectic A-liquid crystals,...) general relativity : approximates Hawking mass elasticity mechanics : nonlinear plate theory. Γ-limit 3d 2d string theory optical design conformal geometry

14 An Analogy Let Γ : [0, 1] R 3 be a closed curve with curvature κ. Let the total curvature w(γ) := 1 0 κ(t)dt Theorem [Fenchel, Borsuk] w(γ) 2π with equality if and only if curve is planar and convex. Theorem [Milnor, Fàry] If w(γ) 4π then Γ is unknotted.

15 Theorem [Willmore, Pinkall] Let Σ be an immersed closed surface. Then W (Σ) 4π and equality holds if and only if Σ is round sphere. Theorem [Li-Yau] Let Σ be an immersed closed surface. If then Σ is embedded. W (Σ) < 8π

16 Willmore Surfaces Critical points of the Willmore energy are called Willmore surfaces. minimal surfaces : H = 0 with W = 0 CMC surfaces (i.e. H = cst) in general not Willmore composition of minimal immersion and conformal transformation The round sphere S 2. Theorem [Bryant] A compact Willmore surface of genus zero in R 3 is inversion of complete minimal surface with finite total curvature and embedded planar ends.

17 The Willmore torus T 0, projection onto R 3 of Clifford torus : ( (θ, ϕ) (cos ϕ + 2) cos θ, (cos ϕ + ) 2) sin θ, sin ϕ) energy W (T 0 ) = 2π 2 Theorem [Marques-Neves] Among all immersed tori in R 3, the Willmore torus is the one which minimizes the Willmore energy (up to isometries of S 3 ).

18 W (Σ) := H 2 da = κ 2 ) Σ 4 Σ(κ 2 = 1 [ (κ1 κ 2 ) 2 ] + 4κ 1 κ 2 4 Σ Gauss-Bonnet theorem : κ Σ 1κ 2 da = 2πχ(Σ) = W (Σ) = 1 (κ 1 κ 2 ) 2 da + 2πχ(Σ) 4 Σ Willmore energy measures sphericity" Gauss equation : 4H 2 2κ 1 κ 2 = d n 2 = W (Σ) = 1 4 Σ d n 2 da + πχ(σ) Willmore energy Dirichlet energy of Gauss map n Also: 2H n = Φ (negative Laplace-Beltrami for induced metric) = W (Σ) = 1 4 Φ 2 da Willmore energy biharmonic energy of immersion Φ Σ

19 The Willmore Equation Theorem [Shadow, Thomsen, Weiner] A smooth immersion Φ is Willmore if and only if gh + 2H (H 2 K ) = 0, g : Laplacian-Beltrami for induced metric. What kind of questions might an analyst ask? meaning of solution (e.g. weak vs. strong) existence (e.g. with or without constraints, which constraints,...) regularity, singularities, compactness

20 The Willmore Equation in Conservative Reformulation Noether: an invariance in the energy gives rise to a conserved quantity. Theorem [Rivière ( 08), Y.B. ( 14)] Translation: T j := H j n n j H + H 2 j Φ satisfies divt j = 0. Rotation: div ( T j Φ ) + H n j Φ = 0 Dilation: div ( T j Φ ) = 0 Last two divergence-free forms integrate to two potentials S and R An unexpected pleasant surprise: g 1/2 gs = x1 n x2r x2 n x1r ( ) ( g 1/2 gr = x1 n x2r x2 n x1r + x1 n x2 S x2 n x1 S ) ( ) ( ) g 1/2 gφ = x1r x2φ x2r x1φ + x1 S x2φ x2 S x1φ

21 Theorem [Y.B., T. Rivière, Annals of Math. 14] Σ arbitrary closed 2-dim manifold. Modulo action of Möbius group of R 3, the space of Willmore surfaces satisfying W (Σ) < 8π δ is compact in the C l topology, l N, δ > 0. Proof relies heavily on Noether s theorem applied to invariances of Willmore energy: Willmore equation is truly a 2 2 system rather than a 4 th -order equation.

22 Open Questions Willmore conjecture in higher codimension? Analogue of the Willmore conjecture for genus 2. Understand the Seifert diagram rigorously Dynamical models Willmore-Plateau problem Replace the Willmore energy for surfaces by H p for p-dimensional Σ Σ