Polarization of electric double layers (due to AC electric-fields) is found to give rise to
various phase/state transitions in concentrated fd- virus suspensions at a low ionic strength.
The phases and dynamical states are observed through an electric cell (see the figure below) by means of depolarized light microscopy (M), electric birefringence (EB), video correlation, and vertical dynamic light scattering (DLS). The photo below gives an overview of the set ups.
~ 200 um
V-DLS
Pitch varianceis measured at different field amplitude at a low
frequency (10Hz)
In-situ electric-cell
Ref. : K. Kang and J. K. G. Dhont, Euro. Phys. Lett. 84, 14005 (2008)
Typical optical morphologies at low frequency (scale bar: ~200um)
Non-equilibrium phase transitions and pattern formation in external electric fields :
EB V_DLS
VM
At low field strengths (and without an external field), the stable phase is a non-chiral nematic (see the upper left image). It is well known that at higher ion strengths, fd-virus suspensions form a chiral phase. At the low ionic strength in our experiments, the Debye screening length is probably so large (27 nm) that the chiral structure of the core of the fd-virus particles is screened. At higher field strength (at a fixed frequency of 10 Hz), chiral nematic is also formed (see the upper right image). The external field thus seems to decrease the range of ectrostatic interactions to an extent that the chiral structure of the cores of fd becomes important. On further increasing the field amplitude, the large non-chiral domains become smaller ans disconnected (lower left image). At even higher field amplitudes, the chiral nematic texture disappears and the non-chiral domains melt and reform (a snap shot is given in the lower right image). The time scale on which melting and reforming occurs rapidly increases on increasing the field strength. At high frequencies, a homogeneous phase is observed, where the rods are aligned along the direction f the external field.
