Soft Matter Blog

1. You recently published a paper re-examining the Archimedes’ principle. What motivated you to take a second look at this?
We were actually trying to understand some rather weird instability phenomena we had observed in the settling of binary colloid mixtures (see J. Phys.: Cond Matt. 24, 284109, 2012 ). Of course, to perform a numerical simulating of sedimentation, one has to give a value for the buoyant force acting on a particle. Yet, if we consider a particle of type 1 settling in a suspension of particles of type 2, this is less trivial than expected: using the standard Archimedes’ principle, what value  for the density  of the “displaced fluid” should we use, that of the bare solvent or that of the  suspension of the host particles? Looking back at the literature, both attitudes can be found. It turns out that neither of the two is correct. 
(Ref. Soft Matter, 2012, DOI 10.1039/C2SM26120K)

2. Can you explain the main points you discovered in this paper?
The crucial point is precisely the expression “displaced fluid”. When a particle in inserted in a complex fluid, or in general in a liquid displaying strong correlations, it does not “displace” just its own volume. Because of its interactions with the solvent, the particle perturbs the local density of the surrounding too. In the example of binary hard-sphere colloids, for instance, a region “depleted” of the host particles forms around particle 1, followed at larger distances, if particles 2 are sufficiently concentrated, by a region where the latter display an oscillating concentration profile. The interesting point is that a the effect of these perturbations on buoyancy can be quantitatively evaluated using a very general expression obtained by extending a simple mechanical equilibrium argument used in elementary physics courses to derive the Archimedes’ principle. This corrected form of the buoyant form, which we call “Generalize Archimedes’ Principle” (GAP), may lead to curious and counterintuitive effects that we tested experimentally.

Read the full interview

8 micrometer bubbles produced by Kumacheva via bubble dissolution.

Microfluidics is a very successful technology widely used to produce droplets with sizes in the range of 10-200 µm. However, the production and behaviour of droplets smaller than this, around 1 µm, has not really been explored. Droplets of this size are highly desirable due, amongst other things, to their biomedical applications.

There are a number of possible methods, which could be used to generate micron sized droplets. Kumacheva et al. formed bubbles of < 10 µm using a dissolution technique. Micro-bubbles of 50-100 µm were formed initially and allowed to carefully dissolve until they reached the desired size. Anna et al., on the other hand, used tip streaming through a flow focusing device to form micron sized liquid drops. While both of these methods have successfully formed droplets smaller than 10 µm, droplet formation using these techniques can be hard to control as both methods rely on specific physiochemical conditions and have relatively low throughputs.

In a recent talk, Patrick Tabeling discussed a brute force method, which he used to form simple droplets, multiple droplets, particles and Janus particles with diameters of 900 nm – 3 µm. The microfluidic devices used by Tabeling et al. contain a submicrometric channel, with a cross junction (where the dispersed and continuous phases meet), followed by a terrace. The interface between the two fluids forms a tongue as it flows down the nanofluidic terrace. The terrace empties into a reservoir, who’s depth is much greater than that of the nanochannel. As the tongue tip crosses into the reservoir it becomes unstable and droplets are formed. Highly monodisperse droplets can be generated using this technique at a rate of 5-15 kHz.

In his talk, Tabeling demonstrated the use of these micron sized droplets for targeted drug delivery in rats. Fluorescence containing droplets were injected into rats, targeted disruption of the droplets and delivery of the fluorescence was achieved using acoustic waves.

A Soft Matter paper by Roberto Piazza and co-workers has been highlighted on a number of Italian news-sites, ANSA, Sole 24 Ore and Galileo.

The paper combines experimental and theoretical studies on what buoyancy actually is, and its relation to the Archimedes’ principle.

What buoyancy really is. A generalized Archimedes’ principle for sedimentation and ultracentrifugation
Roberto Piazza,  Stefano Buzzaccaro,  Eleonora Secchi and Alberto Parola
Soft Matter, 2012, 8, DOI: 10.1039/C2SM26120K

An excerpt of the piece by the Italian on-line magazine, Galileo

This is the way I rewrote Archimedes’ Principle
Sandro Iannaccone | Published June 28, 2012 13:55

It is the Archimedes’ Principle: a body immersed in a fluid receives an upward thrust equal to the weight of the volume of fluid displaced. A definition that all children learn from from school, established since twenty-three centuries. Yet, this time they are the researchers at Politecnico of Milan to proclaim the famous “Eureka!”: their studies have shown that in some cases the principle, in its classical formulation, is not in agreement with the experimental results. The scientists then set out a more general version of this law of physics, presented in this interview with Roberto Piazza, Director of the Laboratory Soft Matter of Politecnico, author of “Soft Matter, the stuff that dreams are made of” (Springer, 2011) and, together with Alberto Parola of the University of Insubria, of this work published in “Soft Matter”.