Mika M Kohonen | PubMed Health

PubMed2006

Engineered wettability in tree capillaries.

The water-conducting network of capillaries in plants has evolved to cope with the frequent occurrence of cavitation, which leads to air-filled capillaries that are unable to function in water transport. However, the material from which the capillaries are constructed is not perfectly wetted by water, contrary to what is often assumed and to what one might expect in an optimal design. I demonstrate that nature is able to overcome this deficiency by engineering the roughness of the capillary walls to achieve near complete wetting, a strategy analogous to that used in the natural engineering of the wettability of plant leaves and insect wings. The results provide an appealing answer to the long-standing debate on the function of wall sculpturing in plant capillaries, and may provide biomimetic clues for the engineering of wettability in technological applications such as microfluidics.

Biomimetics↗

PubMed2003

Mixing and condensation in a wet granular medium.

We have studied the effect of small amounts of added liquid on the dynamic behavior of a granular system consisting of a mixture of glass beads of two different sizes. Segregation of the large beads to the top of the sample is found to depend in a nontrivial way on the liquid content. A transition to viscoplastic behavior occurs at a critical liquid content, which depends upon the bead size. We show that this transition can be interpreted as a condensation due to the hysteretic liquid bridge forces connecting the beads, and we provide the corresponding phase diagram.

Journal Article↗

PubMed2003

Evaporation and instabilities of microscopic capillary bridges.

The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young-Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2r(K)costheta, where r(K) is the Kelvin radius and theta is the contact angle. During the course of slow evaporation (at H > 2r(K)costheta) and subsequent regrowth by capillary condensation (at H < 2r(K)costheta), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.

Chemistry↗