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Deforming glass can be easier than squeezing out water

Recent developments in microfluidics aiming for "lab-on-a-chip" devices require a better understanding of the behavior of fluids flowing in narrow channels. The minimum size above which a confined liquid keeps its bulk behavior has thus been extensively studied and is still open to controversy.

A collaboration between three research groups in Lyon (ILM), Grenoble (LiPhy) and Orsay (LPS) has shown that one can make a fundamental error in the interpretation of experimental data by forgetting that liquids can deform surfaces as rigid as glass. Using a dynamic surface force apparatus (DSFA), they have measured the force required to oscillate a glass ball (3 mm in radius) above a small sheet of glass (1 cm x 1 cm and 5 mm thick) in different liquids (mixtures of water and glycerol, silicone oils). The amplitude of the oscillations of the ball is very small (a fraction of a nanometer) and the selected frequencies of a few tens of Hertz.

Oscillating the ball closer and closer to the glass surface, down to nanometric gaps, they expected the measured force to be purely in phase with the speed of the sphere and to diverge as the sphere–sheet distance decreased to zero. In fact, they measured an apparent viscoelastic behavior of the liquid. In addition, the viscous force did not diverge. A classical Newtonian fluid can thus appear to be non-Newtonian, with strange viscoelastic properties due to the confinement and not governed by the shear rate (see the Figure).

This is just an artifact. It was possible to completely interpret the experimental data for all the investigated liquids by assuming that they keep their mechanical properties down to molecular confinements but taking into account the deformation of the glass when the containment becomes too large. As the liquid gets sufficiently confined, it can be easier to compress glass than to get water to flow!

This work opens new perspectives for understanding the flow of confined liquids, ranging from simple fluids to systems as complex as concentrated solid suspension (mud or concrete) or nanocomposite materials, since in these confined complex fluids the mechanical properties of the surrounding surfaces cannot be neglected.

Apparent viscoelastic modulus G=G’+iG’’ of a water-glycerol mixture of viscosity η = 35 mPa.s measured at 19 Hz between a glass sphere of radius R= 3.59 mm and a plane of the same glass as a function of the sphere-plane distance.The response expected for such a fluid corresponds to G’’ following the horizontal line and G’ = 0. The authors showed that the difference with respect to the expected behavior can be explained by the elastic deformation of the sphere and the glass sheet.


Frédéric Restagno

Effect of Surface Elasticity on the Rheology of Nanometric Liquids
Villey, R., Martinot, E., Cottin-Bizonne, C., Phaner-Goutorbe, M., Léger, L., Restagno, F., and Charlaix, E.
Physical Review Letters, 111, 215701, (2013).
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