Seeking A Low-Energy Position

'The Lord above gave man an arm of iron, so he could do his job and never shirk,' sings Alfred Doolittle in the musical My Fair Lady. 'But with a little bit o' luck,... someone else will do the blinkin' work.' Doolittle was lazy; but the dislike for wasting energy, or more precisely the desire to reach a goal with a minimum investment of energy, is a universal phenomenon common to humans and molecular particles alike. In the case of simple molecules, it drives them to create dense structures (such as solids or liquids), characterized by a large number of bonds between molecules 'costing' a minimum of energy per bond.

But molecular particles (and perhaps people as well) have another, conflicting characteristic that is just as important: under certain conditions they prefer a state of maximum disorder (entropy). The key factor influencing these conflicting tendencies is temperature: at very high temperatures entropy wins and the particles assume a gaseous state, the freest and most disorderly state possible. For example, at high temperatures, water turns to steam. But when the particles are cooled, the energy-saving tendency begins to dominate, causing them to reorganize into more orderly structures. Thus water vapor, when cooled, condenses into a liquid or a solid.

These tendencies apply to simple liquids, called isotropic liquids, composed of particles in which the energy 'cost' of bonding between the particles is independent of their orientation. But what about anisotropic systems, which are made up of particles whose bonding energy is highly dependent on their mutual orientation? Can cooling cause these systems to condense?

This is a fundamental physics question, but one that may also have important applications, since anisotropic liquids play an important role in modern technology. Dr. Tsvi Tlusty and Prof. Sam Safran, his doctoral adviser and Dean of the Weizmann Institute's Feinberg Graduate School decided to tackle this challenge.

In the past, most scientists believed that the answer to this question depended on the nature of the mutual attraction between the particles constituting a liquid. That is, if the mutual attraction is strong enough, it can 'compensate' for the loss of entropy (the loss of disorder involved in the process of condensing matter from gas to liquid).

One of the most striking examples of anisotropic liquids is that of magnetic liquids, characterized by magnetic particles that 'float' within a simple liquid, such as water or oil. In ordinary isotropic liquids, the condensed state of matter is formed by the clustering of particles, each clinging to a relatively large number of other particles (between 6 and 12). In contrast, magnetic liquids in dilute solutions form chains where each particle adheres to only two other particles at most, in a north-south-north-south structure. The bonding energy between these particles is highly dependent on their magnetic orientation. When two such magnets are aligned (i.e. their poles are positioned north to north or south to south) they repel each other; but when the nearby poles are of opposite orientation, the magnets mutually attract - which results in the north-south pole structure. This chaining structure in magnetic liquids prevents the usual type of condensation; because each of the particles is in contact with only two of its neighbors, it has less bonding energy. Therefore, until recently, the accepted scientific wisdom was that magnetic liquids could not undergo the usual gas-to-liquid condensation.

Safran and Tlusty's research sheds new light on this belief. Using a theoretical model, the scientists have shown that magnetic liquids can undergo condensation. Their conclusion seems from the fact that the magnetic chains 'prefer' to form Y-like junctions that bring together three chains (the energy 'cost' of this state is lower than the 'cost' of having both ends of a chain free). Thus magnetic chains in a dilute solution tend to form large and complex networks. In their study, recently published in Science, the researchers suggest that these network junctions strive for a balance between a state that 'costs' a minimum of energy and a state of maximum entropy. The network finds it 'worthwhile' to create more and more junctions, thus increasing its entropy while making the network denser and more complex. When the system of junctions making up the network is dilute, the material is in the gaseous phase; but when the network becomes more dense, the substance condenses to increase the junction entropy and makes the transition from a gas to a liquid.

This finding overturns the previously accepted view that the condensation of magnetic liquids is impossible without the involvement of additional, isotropic forces. In these systems, it is actually the increase in network entropy that stabilizes the condensed liquid state. No other forces are necessary. Further experimentation, resulting in confirmation of the scientists' 'network model,' might lead to the emergence of an entirely new field of technological applications. For instance, a better understanding of the physical properties of anisotropic liquids, and especially of their great sensitivity to changes in magnetic or electric fields, is important for developing advanced computers and other micro-machines.

Prof. Safran holds the Fern and Manfred Steinfeld Professorial Chair.