Helical Defect Packings in a Quasi-One-Dimensional System of Cylindrically Confined Hard Spheres

We use a combination of analytical theory and molecular dynamics simulation to study the inherent structure landscape of a system of hard spheres confined to narrow cylindrical channels of diameter 1 + root 3/2 < H-d/sigma < 1 + 4 root 3/7. The most dense packing is a perfect, symmetrical single helix. The presence of topological defects that change the local chirality of the helix lowers the packing density and alters the local packing structure. The helical sections between defects become asymmetrical and are better described as a double helix with angular twists between the first and second nearest neighbors that are determined by the defect separation distance. Increasing the fraction of defects unwinds the two helical strands so that the least dense structure is a nonhelical packing of two zigzag chains. We also show that the packing effects of the helical section induce a long-range, entropically driven attraction between the defects.