Liquid crystals
Fluids endowed with internal orientational order, such as liquid crystals, might possess defects not only in the flow velocity, but also in the average orientation of their molecular building blocks. These orientational defects, known as disclinations, are among the most characteristic and intensively researched features of liquid crystals. Leiden physicist Luca Giomi is one of those researchers. ‘Display makers hate those defects and spend fortunes on expensive glass-rubbing machines to get rid of them. Soft matter physicists, on the other hand, love these tiny singularities and try to device ways to exploit them.’
Cross-talk
In a paper recently published in PNAS, Giomi and collaborators describe their discovery of an intriguing "cross-talk" between defects in different material fields: the fluid velocity and the average molecular orientation. Giomi: ’Defects having the same rotational symmetry attract each other even if they belong to different fields. So a stagnation point in a two-fold symmetric flow (center-top panel, edit.) acts as a tweezer for a two-fold disclination (right-top panel, edit.).’ Stagnation points in flows with higher symmetry (i.e. three-fold, four-fold etc.) attract more complicated disclinations where the orientation winds around the defect core more than once (lower panels).
Currents in a liquid crystal, both experimentally (center row) and theoretically (right row). The red arrows indicate the inflowing currents; the green arrows the outflowing currents. The yellow markings in the right row indicate the stagnation points: the ‘defects’. With currents coming from more than two directions, multiple defects are formed.
Credits
This research was conducted by Luca Giomi (Leiden), Anupam Sengupta (ETC Zurich), Žiga Kos and Miha Ravnik (Ljubljana).