Evolution of ZDDP-derived reaction layer morphology with rubbing time.

Functional additives, particularly extreme pressure and antiwear additives, in formulated oil will compete to adsorb and function in tribological contacts. A low-polarity commercial base oil, poly-α-olefin (PAO), blended with zinc dialkyl dithiophosphates (ZDDP) has been studied. The tribological performance was evaluated using a ball-on-disk test rig under mixed rolling-sliding conditions in the boundary lubrication regime at 90°C. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The thickness of the reaction layer evolves with rubbing until reaching a limiting thickness value of approximately 70 nm. The evolution of the topography and mechanical properties of the ZDDP-derived reaction layer with rubbing time were studied using Atomic Force Microscopy. A constant roughening and hardening of the additive-derived layer with rubbing time is observed and related to the different tribological performance of the layer at different rubbing times.

Creation of nanohillocks on CaF2 surfaces by single slow highly charged ions.

Upon impact on a solid surface, the potential energy stored in slow highly charged ions is primarily deposited into the electronic system of the target. By decelerating the projectile ions to kinetic energies as low as 150 x q eV, we find first unambiguous experimental evidence that potential energy alone is sufficient to cause permanent nanosized hillocks on the (111) surface of a CaF(2) single crystal. Our investigations reveal a surprisingly sharp and well-defined threshold of potential energy for hillock formation which can be linked to a solid-liquid phase transition.

Atomic force microscopy study of living diatoms in ambient conditions.

We present the first in vivo study of diatoms using atomic force microscopy (AFM). Three chain-forming, benthic freshwater species -Eunotia sudetica, Navicula seminulum and a yet unidentified species - are directly imaged while growing on glass slides. Using the AFM, we imaged the topography of the diatom frustules at the nanometre range scale and we determined the thickness of the organic case enveloping the siliceous skeleton of the cell (10 nm). Imaging proved to be stable for several hours, thereby offering the possibility to study long-term dynamic changes, such as biomineralization or cell movement, as they occur. We also focused on the natural adhesives produced by these unicellular organisms to adhere to other cells or the substratum. Most man-made adhesives fail in wet conditions, owing to chemical modification of the adhesive or its substrate. Diatoms produce adhesives that are extremely strong and robust both in fresh- and in seawater environments. Our phase-imaging and force-pulling experiments reveal the characteristics of these natural adhesives that might be of use in designing man-made analogues that function in wet environments. Engineering stable underwater adhesives currently poses a major technical challenge.

Probing protein-protein interactions in real time.

We have used a prototype small cantilever atomic force microscope to observe, in real time, the interactions between individual protein molecules. In particular, we have observed individual molecules of the chaperonin protein GroES binding to and then dissociating from individual GroEL proteins, which were immobilized on a mica support. This work suggests that the small cantilever atomic force microscope is a useful tool for studying protein dynamics at the single molecule level.

Micromechanical models for the Brownian motion of hair cell stereocilia.

Brownian motion of the hairs (stereocilia) of amphibian hair cells has been shown in experiments to be in the range of some nm. Our models of the Brownian motion of coupled harmonic oscillators with mechanical properties of stereocilia lead to similar displacements. Computer simulation shows that stochastic fluctuations enhance the encoding of low level acoustic signals. Stochastic resonance lowers the detection threshold of auditory signals to amplitudes one order of magnitude lower than that of the Brownian motion.

The mammalian auditory hair cell: a simple electric circuit model.

A model based on the potassium current pathway through the hair cell is used to analyze the electrical behavior of mammalian inner and outer hair cells. Without taking into account the effects of calcium it is possible to simulate experimental results concerning the shape and strength of the receptor potential and the frequency dependent ac (alternating current) and dc (direct current) components of the receptor current. This model and a simplified form of it are utilized to explain: (1) Transduction latencies: that the receptor potential follows a stimulating signal with a very short delay, under the assumption of a constant number of open K+ channels in the lateral part of the cell membrane. (2) Transduction gains: why higher potential changes are measured in inner hair cells than in outer hair cells, although the outer hair cells are expected to be exposed to higher stereociliary motions: in inner hair cells a decrease in the conductance of the basolateral membrane causes higher gain (receptor potential increases) and together with an increase of membrane capacitance slower reaction (a larger time constant). (3) Transduction channel kinetics: that the shortest (0.1 ms) as well as the longest (20 ms) possible open times of the transduction channels in the stereocilia have different frequency related effects on the shape of the receptor potentials.