Monday, 23 January 2012

Twisting interfacial tension...

The understanding of science and surface tension in particular are one of those things where you think something as old as Benjamin Franklin (who conducted the first studies) would be understood to its fullest potential.  So I am always learning something new about surface science and surface tension in general.  I stumbled upon this that the Dogic group at Bradeis University in Massachusetts has demonstrated a demonstrated a new way of controlling interfacial tension using a change in chirality of a condensed membrane of rod-like bacteriophage virus fd.  By increasing the strength of chirality of the rods it both lowers the energy of the rods along the membrane’s edge and increases the frustration of untwisted rods in the bulk, lowering the interfacial tension  They published this in some unknown magazine called Nature (at least publish in Phys Letters). Here a link to the 2012 article.  (Read then watch these cool movies below)

Let's back up here and explain a few things.  Water has a surface tension of 72 dynes/cm2 which allows many things like water striders and even the ocassional son of God to walk on top of it.  Water also has an another interesting property when you add oil to it.  Oil and water as you have probably figured out do not mix.  Oil will create an interface with the water making a strong interfacial tension (cell membranes are also like this).  Adding soaps or detergents will adjust the surface tension and these soaps which are both hydrophobic and hydrophilic (amphipathic) will move to this interface between the oil and water.  This is basically how your detergents like Tide clean all the stains out of your clothes or SDS can detroy cell membranes.

The Dogic group has made it so that the membrane instead of an outside amphiphile changing the state of teh membrane they changed it using chirality.  Instead of lipid membrane they made a membrane of rod-liek bacteriophage virus fd.  This is a chiral molecule.  Think of your hands being mirror images but cannot superimpose on one another.  Since the rods have this property of chirality they tend to twist in a small angle with respect their neighbors.  When they increased the strength of the chirality it lowers the energy of the rod's along the membrane's edge thus lowering the interfacial tension. 

The interesting thing here is that they can change the microscopic shape of this membrane making it form from a flat two dimensional shape to a one-dimensional ribbon.  This is a fully reversible transition that you can see in the movies on youtube.  Twistier structures have more chiral strength.  In a lipid membrane the change in interfacial tension can be done chemically (by adding different lipids), mechanically or by changing the temperature but it is not always reversible and some degree of hysteresis may occur.

This movie depicts the reversible transition from a 2D membrane to a 1D twisted ribbon by the application of a stretching force imparted by optical tweezers. The diameter of the unstretched membrane is ~10microns.(text taken from youtube)

This movie shows the reversible transition of a 2D colloidal membrane composed of fd viruses into several connected 1D twisted ribbons, induced by increasing the strength of chiral interactions between the constituent virus particles. The duration of the movie is 11.9 minutes. The diameter of the membrane is ~10microns.(text taken from youtube)