tutorial4nematic.morpho

/** Tutorial example: Nematic droplet on a substrate 
    for DSOFT Workshop March 2023 
    Uses Morpho 0.5.6 
    by T. J. Atherton and the softmattertheory group at Tufts University */

import meshgen 
import plot 

var sigma = 20      // Isotropic surface tension fluid-air
var deltasigma = -4 // Difference between surface tension of air-substrate interface and fluid-substrate interface
var wupper = 1      // Anchoring energy on upper surface
var wlower = 1      // Anchoring energy on lower surface
var k = 0.1         // Elastic constant for nematic 

/* Part 1: Create and visualize the Mesh */
var c = CircularDomain([0,0], 1)                            // The unit disk 
var hs = HalfSpaceDomain(Matrix([0,0]), Matrix([0,1]))      // Half space below the origin
var dom = c.difference(hs)                                  // Take the difference
var mg = MeshGen(dom, [-1..1:0.2, -1..1:0.2], quiet=true)   // Mesh this domain
var mesh = mg.build()                                       // Build the Mesh 

/* Part 2: Create and visualize Selections */
var bnd = Selection(mesh, boundary=true)                    // Select boundary of the mesh 
var upper = Selection(mesh, fn (x,y) x^2+y^2>0.99)          // Select the outer boundary of the disk 
upper.addgrade(1)                                           // Add lines (grade 1) elements to the upper selection
var lower = Selection(mesh, fn (x,y) y<0.01)                // Select the lower boundary of the mesh 
lower.addgrade(1)                                           // Add lines (grade 1) elements to the lower selection

/* Part 3: Formulate and solve the isotropic problem */
var problem=OptimizationProblem(mesh)                       // Create an OptimizationProblem with our mesh as the target

var llength = Length()                                      // Apply an Length functional to the upper and lower boundaries 
problem.addenergy(llength, selection=upper, prefactor=sigma) 
problem.addenergy(llength, selection=lower, prefactor=deltasigma)  

var larea = Area()                                          // Add an Area functional as a global constraint
problem.addconstraint(larea) 

var lcons = ScalarPotential(fn (x,y) y)                     // Use a local level set constraint to pin
problem.addlocalconstraint(lcons, selection=lower)          // the lower vertices to the substrate

// Now to solve it 
var opt = ShapeOptimizer(problem, mesh)                     // Create the ShapeOptimizer
opt.conjugategradient(500)                                  // Perform conjugate gradient

/* Part 4: Now add in and solve the nematic problem */
var nn = Field(mesh, Matrix([1,0,0]))                       // Create the director field 

// Helper function to visualize the director field 
fn visdirector(m, n, scale) {
    var g = Graphics() 
    for (id in 0...m.count()) {
        var x = m.vertexposition(id)        // Get vertex id's position 
        var xx = Matrix([x[0], x[1], 0])    // Promote it to a 3D vector 
        var nn =n[0,id]                     // Get the corresponding director 
        g.display(Cylinder(xx-scale*nn, xx+scale*nn, aspectratio=0.2, color=White)) 
    }
    return g
} 

// Now add the nematic terms into the problem 
var lnem = Nematic(nn)                      // Create and add in nematic elasticity 
problem.addenergy(lnem, prefactor = k) 

fn anchintegrand(x, n) { // Integrand for anchoring energy 
    var t = tangent()    // Gets the local tangent vector 
    return (n[0]*t[0]+n[1]*t[1])^2
}

var lanch = LineIntegral(anchintegrand, nn) // Anisotopic surface tension (anchoring)
problem.addenergy(lanch, selection=lower, prefactor=-wupper) 
problem.addenergy(lanch, selection=upper, prefactor=-wlower) 

var lnorm = NormSq(nn)                      // Unit vector constraint 
problem.addlocalconstraint(lnorm, field=nn, target=1) 

var fopt = FieldOptimizer(problem, nn)      // Create the Field Optimizer
fopt.steplimit = 0.1                        // Prevent field optimizer from going too far off constraint space

for (i in 1..30) {                          // Alternating optimization scheme for Shape and Field
    opt.conjugategradient(10)
    fopt.conjugategradient(10)
    if (opt.hasconverged() && fopt.hasconverged()) break 
}

// Visualize 
Show(plotmesh(mesh, grade=[0,1])+visdirector(mesh, nn, 0.1))