Home Research
Elena Pierro - Research Interests
Dr. Pierro's research activities are focused on vibration phenomena and contact mechanics in micro and nano devices, with particular attention to fluid structure interactions. Some investigations:

Atomic Force Microscopes

dAFM are employed in many applications, e.g. to study biological targets as cells, proteins, DNA. They consist of an oscillating microcantilever which holds a sharp nanoscale tip that intermittently interacts, close to its first resonance frequency, with the sample.The micro-cantilever tip often needs to operate in a liquid environment to extract the required information from the sample. However, tip dynamics is strongly affected by the presence of the liquid itself, so that understanding the actual microcantilever response in such conditions, has become one of the most challenging problems the researchers are trying to face. A deep knowledge of the degree of interaction between the cantilever dynamics and the fluid is extremely important to avoid misleading information. Because of the micro-scale size of the cantilever, thermal noise due to Brownian forcing of liquid particles cannot be neglected and therefore proper insights about this effect are required. This is why different numerical approaches have been presented in literature, which only approximatively describe the liquid - cantilever interaction. In this context it has been presented an analytical heuristic formulation of the force the liquid exerts on the cantilever, which can be successfully utilized to investigate the AFM cantilever dynamics under the action of both linear and non linear forces. It has been shown that the liquid response consists of three terms: (i) a viscous term, (ii) a velocity-diffusive term, and (iii) an inertial term. The novelty of the model is mainly represented by the velocity-diffusive term, that to the best of our knowledge has never been taken into account before. We show indeed, that neglecting this term leads to large errors in the estimation of the cantilever response and , hence, of its thermal response, which is often used to calibrate the instrument.

Contact mechanics

Real surfaces are affected by roughness at different lenght scales, and very often a rough surface can be described as a self-affine fractal. In this framework, the influence of fractal dimension on adhesion is investigated. It is found that at high loads the influence of the fractal dimension Df is not negligible. However at small loads the influence of Df is less important in agreement with the predictions of some contact mechanics theories.
Moreover, the adhesion properties of biological fibrillar attachment systems is studied. In particular, it has been recently experimentally proved that mushroom-shaped micro-pillars (Figure 1) have a significantly enhanced adhesion, if compared to simple flat micro-punches. We have carried out a theoretical investigation aimed to clarify the physical mechanisms that determine such a different behaviour. The performance of the two geometries have been compared, in terms of the pull-off force needed to detach the pillar from the substrate. In the flat punch case, we have shown that in almost every practical applications, the detachment occurs because of crack propagation from the edge towards the center of the contact. For the mushroom pillar, on the contrary, the separation at the edge is inhibited by the presence of the terminal plate, and the detachment is a consequence of the propagation of inner defects at the interface. In particular, the presence of the plate eliminates the square root stress singularity (Figure 2-a) (which is a characteristic of the flat punch interfacial stress distribution), therefore it is possible to assert that the plate does not give any significant contribution in supporting the load, but it stabilizes defects at plate-substrate interface. Our pull-off force calculations are in good agreement with some available experimental outcomes, and confirm that mushroom-shaped pillars outperform the flat punch in terms of adhesion strength.
Figure 1: The mushroom shaped pillars. The terminal plate (b) is the origin of the enhanced adhesive performances between of such microstructured surfaces
Figure 2: Stress distribution in case of a flat punch (a) and for a mushroom shaped pillar, for three different thickness of the plate, thin (b), medium (c) and thick (d). The presence of the plate eliminates the stress singularity of the flat punch at r=R. Stress peak in the mushroom pillar at r=R will gradually vanish as the plate thickness t is increased up to its optimal value (c).


Room: +39 080 596 2817

Fax: +39 080 596 2777

Email: e.pierro@poliba.it

Copyright © 2010 --- TriboLAB
DIMEG - Politecnico di Bari,  Viale Japigia, 182 - 70126 BARI (Italy)

Tel. +39 080 596 2746, Fax +39 080 596 2746/2777

Contact us at tribolab.poliba@gmail.com