SCHOOL OF MECHANICAL ENGINEERING SEMINAR Monday, June 4, 2012 at 15:00 Wolfson Building of Mechanical Engineering, Room 206

M.Sc. student of Prof. Slava Krylov and Dr. Rivka Gilat School of Mechanical Engineering, Tel-Aviv University

The asymmetric buckling of curved micro beams subjected to nonlinear deflection-dependent distributed electrostatic force is studied. The structure is described in the framework of the geometrically nonlinear shallow Euler-Bernoulli beam model; the analysis is based on two degrees of freedom reduced order (RO) model resulting from the Galerkin decomposition with linear undamped eigenmodes of a straight beam used as the base functions. Simple approximate expressions are derived defining the geometric parameters of the beam for which an asymmetric response bifurcates from the symmetric one. Two different conditions are defined: the necessary criterion establishes the conditions for the appearance of bifurcation points on the unstable branch of the symmetric limit point buckling curve; the sufficient criterion assures a realistic asymmetric buckling bifurcating from the stable branches of the curve. It is shown that while the symmetry breaking conditions are affected by the nonlinearity of the electrostatic force, its influence is less pronounced than in the case of the symmetric snap-through criterion. A comparison between the results of the RO analysis, and those obtained numerically confirms the applicability of the RO model and the resulting symmetry breaking criteria in design procedures of bistable micro devices. In addition, preliminary experimental results are consistent with the model predictions.

M.Sc. student of Dr. Hadas Mamane School of Mechanical Engineering, Tel-Aviv University

The disinfection agents commonly used both in drinking water and wastewater treatment plants are chlorine and its related compounds. However, they react with the natural organic matter present in water to produce disinfection by-products (DBPs), some of which have been found to be carcinogenic. Thus, ongoing research focuses on the development of alternative disinfection methods. In recent years, advanced oxidation processes (AOPs) have been successfully employed mostly for the oxidation but also for the disinfection of water and wastewaters. The present study examines the feasibility of a novel bismuth - based photocatalyst, an AOP, for water disinfection using Escherichia coli (E. coli) as indicator bacteria for water quality control. The inactivation of E. coli was tested with different photocatalyst concentrations under a solar simulator. The optimal concentration was found to be 0.25 g/L, with a 4 log removal after 80 minutes of irradiation, compared to no removal under solar irradiation only. Various measures were examined in order to understand the mechanism of oxidative stress on E. coli. Such measures included colony forming ability, membrane damage, lipid peroxidation and DNA damage. Results show that bismuth-induced stress attacks the membrane and leads to the loss of membrane potential in E. coli cells. An increase of endonuclease-sensitive sites (ESS), which indicate DNA damage, was observed with the addition of bismuth. The new bismuth- based photocatalyst shows high potential as alternative disinfection treatment of water.

Symmetry breaking criteria in electrostatically loaded curved bistable micro beams

Lior Medina