SUSPENSIONS OF NANOWIRES, NANOTUBES AND VIRUSES

 

A colloidal solution is a suspension of particles so small and light that do not settle in water under gravity effect. Indeed, the Brownian movement of water molecules is enough to keep the colloid in suspension for long times. Examples of colloidal state are ubiquitous in every day life (milk, for instance, is a suspension of fat in aqueous phase), and are involved in several chemical industrial applications (glues, paints, ink, cosmetics, pharmaceuticals, foodstuffs, cements…).

The surface area per gram of a colloidal suspension is very large and this fact is the basic origin of most of the very peculiar behavior of colloidal systems. Strikingly, the properties of the colloids are almost completely independent of the characteristics of the bulk material originating the colloids itself. A colloid is not simply a fine nanodispersed material, it is a completely new world, that’s why is customary to indicate it as: "Colloidal State of Matter".

 

Figure 1: Transmission Electron Microscope picture of the PTFE latex particles.

 

Usually the colloidal particles also have a charge due to ions adsorbed on the surface. This charge prevents aggregation of the particles and therefore coagulation can be induced by neutralizing the charge (by adding some simple electrolyte, like NaCl, for example). Due to inter-particle interactions (electric interactions, for instance), colloids show interesting phase transitions between gas, liquid, solid (colloidal crystals) and liquid crystalline phases. These systems often become undercooled, supersaturated, or trapped in gel-like states.

The aims of colloid science is describing in detail the microscopic structure and composition of the colloid, understanding the origin and the consequences of colloidal interaction, and unraveling the mechanisms of phase transitions.

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COLLOIDAL ROD-LIKE PARTICLES

UNDER THE EFFECT OF EXTERNAL ELECTRIC FIELDS

Despite the continuous investigation by generations of scientists, the microworld of colloidal particles suspended in a liquid still keeps surprising the scientific community with new unexpected behavior. For example, studies of colloids under the effects of external electric fields (electrokinetic effects) have shown a large ensemble of intriguing phenomena. Indeed, after decades of substantial theoretical effort, a clear understanding of the behavior of colloids in external fields is only available for the electric polarization and the migration of diluted spheres in low fields. The interpretation of the electric response of charged rod-like particles is far less advanced. Qualitatively it is easy to envisage that charged particles of rod-like shape (such as viruses and bacteria, together with many other man-made elongated colloids, see fig. 1), when subjected to an electric field, universally orient along the field direction (see fig 2b).

 

 

Recent observations developed in our group indicate that the same rod-like particles, no matter how dilute they are, when mixed with smaller spherical particles bearing charges of the same sign, orient perpendicularly to the field (see fig 2c). This is a striking "anomalous behavior" which characterizes with absolute generality mixtures of charged rod-like and spherical colloids in dilute suspensions. This effect, unexplainable with currents theories, reveals the existence of new forms of unforeseen interparticles interactions.

After our seminal report [Mantegazza, Jimenez, Caggioni, and Bellini, Nature Physics, 1, 103, (2005)] of the "anomalous" behavior in bidisperse mixture of colloidal rod and spheres, we are persisting in investigating both the zoology of examples of "anomalous" orientation and the possible explanations of the effect. Actually, new kind of rod-like particle (microtubules, virus, nanowires and others) in external electric field are under study. On the other side, in collaboration with theoreticians, simulations and new calculations schemes are examined.