Mika M. Kohonen

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, 2rKcosθ, where rK is the Kelvin radius and θ is the contact angle. During the course of slow evaporation (at H > 2rKcosθ) and subsequent regrowth by capillary condensation (at H < 2rKcosθ), 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.

Capillarity at the nanoscale: an AFM view

We have used atomic force microscopy (AFM) to image liquid droplets on solid substrates. The technique is applied to determine the contact line tension. Compared to conventional optical contact angle measurements, the AFM extends the range of accessible drop sizes by three orders of magnitude. We analyze the global shape of the droplets and the local profiles in the vicinity of the contact line. These two approaches show that the optical measurement overestimates the line tension by approximately four orders of magnitude.

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.

On the Function of Wall Sculpturing in Xylem Conduits

The water-conducting network of capillaries in vascular plants has evolved over hundreds of millions of years in order to be able to cope with bubble clogging, a problem which also affects modern microfluidic devices. Decades of anatomical studies have revealed that plants growing in habitats in which the formation of bubbles, or emboli, is likely to be a frequent occurrence often have various forms of geometrical sculpturing on the internal surfaces of the xylem conduits. The possible function of such wall sculpturing has long been the subject of speculation. We have investigated the hypothesis that wall sculpturing is a functional adaptation designed to increase the wettability of the walls of xylem conduits, an effect which could be described as the inverse of the well-known lotus-effect. Our results show that wall sculpturing does enhance wettability. Importantly, theoretical calculations reveal that the geometric parameters of various types of wall sculpturing are such that the resulting surfaces are sufficiently rough to enhance wettability, but not significantly rougher. The results provide an appealing answer to the long-standing debate on the function of wall sculpturing in xylem conduits, and may provide biomimetic clues for new approaches to the removal of bubbles in microfluidic channels.

The influence of surface energy on the self-cleaning of insect adhesive devices

SUMMARY The ability of insects to adhere to surfaces is facilitated by the use of adhesive organs found on the terminal leg segments. These adhesive pads are inherently ‘tacky’ and are expected to be subject to contamination by particulates, leading to loss of function. Here, we investigated the self-cleaning of ants and beetles by comparing the abilities of both hairy and smooth pad forms to self-clean on both high and low energy surfaces after being fouled with microspheres of two sizes and surface energies. We focused on the time taken to regain adhesive potential in unrestrained Hymenopterans (Polyrhachis dives and Myrmica scabrinodis) and Coccinellids (Harmonia axyridis and Adalia bipunctata) fouled with microspheres. We found that the reattainment of adhesion is influenced by particle type and size in Hymenopterans, with an interaction between the surface energy of the contaminating particle and substrate. In Coccinellids, reattainment of adhesion was only influenced by particle size and substrate properties. The adhesive organs of Coccinellids appear to possess superior self-cleaning abilities compared with those of Hymenopterans, although Hymenopterans exhibit better adhesion to both surface types.

Use of the light-lever technique for the measurement of colloidal forces

Abstract Force measurements in a variety of colloidal systems have been conducted using a new device that utilises the light-lever technique employed in Atomic Force Microscopy. The systems studied include electrostatic double-layer forces, hydrophobic interactions and liquid crystal structures. The new device has some clear advantages over adaptation of an imaging AFM for colloidal force measurements.

Effect of particulate contamination on adhesive ability and repellence in two species of ant (Hymenoptera; Formicidae)

SUMMARY Tarsal adhesive pads are crucial for the ability of insects to traverse their natural environment. Previous studies have demonstrated that for both hairy and smooth adhesive pads, significant reduction in adhesion can occur because of contamination of these pads by wax crystals present on plant surfaces or synthetic microspheres. In this paper, we focus on the smooth adhesive pads of ants and study systematically how particulate contamination and the subsequent loss of adhesion depends on particle size, particle surface energy, humidity and species size. To this end, workers of ant species Polyrhachis dives and Myrmica scabrinodis (Hymenoptera; Formicidae) were presented with loose synthetic powder barriers with a range of powder diameters (1–500 μm) and surface energies (PTFE or glass), which they would have to cross in order to escape the experimental arena. The barrier experiments were conducted for a range of humidities (10–70%). Experimental results and scanning electron microscopy confirm that particulate powders adversely affect the adhesive ability of both species of ant on smooth substrates via contamination of the arolia. Specifically, the loss of adhesion was found to depend strongly on particle diameter, but only weakly on particle type, with the greatest loss occurring for particle diameters smaller than the claw dimensions of each species, and no effect of humidity was found. We also observed that ants were repelled by the powder barriers which led to a decrease of adhesion prior to their eventual crossing, suggesting that insect antennae may play a role in probing the mechanical fragility of substrates before crossing them.