Elastic and hydrodynamic torques on a colloidal disk within a nematic liquid crystal

Abstract

The orientationally dependent elastic energy and hydrodynamic behavior of colloidal disks with homeotropic surface anchoring suspended in the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) have been investigated. In the absence of external torques, the disks align with the normal of the disk face â parallel to the nematic director n[over ^]. When a magnetic field is applied, the disks rotate â by an angle θ so that the magnetic torque and the elastic torque caused by distortion of the nematic director field are balanced. Over a broad range of angles, the elastic torque increases linearly with θ in quantitative agreement with a theoretical prediction based on an electrostatic analogy. When the disks are rotated to angles θ>π/2, the resulting large elastic distortion makes the disk orientation unstable, and the director undergoes a topological transition in which θ→π-θ. In the transition, a defect loop is shed from the disk surface, and the disks spin so that â sweeps through π radians as the loop collapses back onto the disk. Additional measurements of the angular relaxation of disks to θ=0 following removal of the external torque show a quasi-exponential time dependence from which an effective drag viscosity for the nematic can be extracted. The scaling of the angular time dependence with disk radius and observations of disks rotating about â indicate that the disk motion affects the director field at surprisingly modest Ericksen numbers.

Hindered nematic alignment of hematite spindles in poly(N-isopropylacrylamide) hydrogels: a small-angle X-ray scattering and rheology study

“…As early as 1949, Onsager predicted the formation of a nematic phase by mineral colloids, with the decrease in excluded volume compensating for the loss of orientational entropy (Onsager, 1949). The dynamic behaviour of discotic colloidal particles in the shear flow of a Newtonian liquid (Lettinga et al, 2012) and in the nematic phase of 5CB (Rovner et al, 2012) have been investigated. The phase behaviour of ellipsoidal magnetic particles consisting of silica-coated hematite particles in an external magnetic field was investigated by Martchenko et al (2016).…”

Star colloids in nematic liquid crystals

“…We note that if n(r) singularities such as line disclinations or point defects are necessary to transition from the metastable branch to the ground state branch, as is the case for externally torqued axially-symmetric bodies such as cylinders 66 and disks 59 in NLCs, then the appropriate core energies of the defects would also contribute to DU el .…”

Theory and simulation of objects in liquid crystals

“…The approximations required for these calculations do not hold when the object is twisted to a large angle θ. At this point, the energy can be decreased by moving out of plane and the object spontaneously 'flips' [52,59,60]. We will discuss this further in a later section.…”

“…We now return to the case of the magnetic disc in a liquid crystal discussed in Section 5. In this case, the disc can be rotated within the liquid crystal by changing the external magnetic field [29,52,60]. When the disc is rotated to a high angle it becomes unstable and 'flips'.…”