Complex Fluids
fd Virus — a model system of rod-like colloids
We use fd virus as a model system of colloidal rods. Bacteriophage fd is a good model system
to study liquid crystalline behavior bacause it is easily grown in large quantities, it is very monodisperse and it is very
stable in solution. In addition, because fd infects E.Coli. it is easy to use standard molecular cloning techniques to alter the basic
properties of the virus.
The above image is an electron micrograph of an fd virus. fd wt is a long semi-flexible rod with
contour length of 880 nm, diameter of 7 nm and persistence length of 2.2 µm. It is highly charged which results in a very stable
colloidal suspension.
At concentrations around 20 mg/ml we observe a coexistence between isotropic and
nematic/cholesteric phase. The isotropic phase has short range positional and orientational order and appears dark
between crossed polarizers. The anisotropic phase has long range orientational order and is birefriengent under crossed
polarizers.
We find that fd virus form a cholesteric phase instead of the nematic phase where the nematic director
forms a large helical structure. The microscopic origin of the cholesteric phase is not understood as is explained in the following paper
by .
At very high concentrations we observe formation of the smectic phase. The periodic pattern is due to the
smectic layers which are composed of a two dimensional liquid of essentially aligned rods. Due to large contour length of the
virus it is possible to directly observe smectic layers with optical microscope. Using well established molecular cloning techniques
we are able to systematically alter the contour length of the virus. The above image show four different smectic phases in which
the viruses contour length varies between 0.4 and 1.2 µm. More detail about the phase diagram of fd virus is
available in the following paper by . More
details about fd virus is available at Seth Fraden's website:
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