Effective interactions of colloids on nematic films

The elastic and capillary interactions between a pair of colloidal particles trapped on top of a nematic film are studied theoretically for large separations $d$. The elastic interaction is repulsive and of quadrupolar type, varying as $d^{-5}$. For macroscopically thick films, the capillary interaction is likewise repulsive and proportional to $d^{-5}$ as a consequence of mechanical isolation of the system comprised of the colloids and the interface. A finite film thickness introduces a nonvanishing force on the system (exerted by the substrate supporting the film) leading to logarithmically varying capillary attractions. However, their strength turns out to be too small to be of importance for the recently observed pattern formation of colloidal droplets on nematic films.Comment: 13 pages, accepted by EPJ

Free energy of colloidal particles at the surface of sessile drops

The influence of finite system size on the free energy of a spherical particle floating at the surface of a sessile droplet is studied both analytically and numerically. In the special case that the contact angle at the substrate equals $\pi/2$ a capillary analogue of the method of images is applied in order to calculate small deformations of the droplet shape if an external force is applied to the particle. The type of boundary conditions for the droplet shape at the substrate determines the sign of the capillary monopole associated with the image particle. Therefore, the free energy of the particle, which is proportional to the interaction energy of the original particle with its image, can be of either sign, too. The analytic solutions, given by the Green's function of the capillary equation, are constructed such that the condition of the forces acting on the droplet being balanced and of the volume constraint are fulfilled. Besides the known phenomena of attraction of a particle to a free contact line and repulsion from a pinned one, we observe a local free energy minimum for the particle being located at the drop apex or at an intermediate angle, respectively. This peculiarity can be traced back to a non-monotonic behavior of the Green's function, which reflects the interplay between the deformations of the droplet shape and the volume constraint.Comment: 24 pages, 19 figure

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund

Generating the Hopf fibration experimentally in nematic liquid crystals

Article / Letter to editorLeids Instituut Onderzoek Natuurkund