Thermoacoustic effects in supercritical fluids near the critical point: Resonance, piston effect, and acoustic emission and reflection

We present a general theory of thermoacoustic phenomena in supercritical fluids near the critical point in a one-dimensional cell. We take into account the effects of the heat conduction in the boundary walls and the bulk viscosity near the critical point. We introduce a coefficient $Z(\omega)$ characterizing reflection of sound with frequency $\omega$ at the boundary. As applications, we examine the acoustic eigenmodes in the cell, the response to time-dependent perturbations, sound emission and reflection at the boundary. Resonance and rapid adiabatic changes are noteworthy. In these processes, the role of the thermal diffusion layers is enhanced near the critical point because of the strong critical divergence of the thermal expansion.Comment: 15 pages, 7 figure

Evaluation of the Antioxidant Capacity of Fruit Juices by Two Original Analytical Methods

Two analytical methods previously developed by our groups were employed to estimate the antioxidant capacity of commercial fruit juices. The electrochemical method, which measures the scavenging activity of antioxidants towards OH radicals generated by both hydrogen peroxide photolysis and Fenton’s reaction, is based on the recovery of the cyclic voltametric response of the redox probe Ru(NH3)63+ at a Glassy Carbon electrode modified with a thin film of an insulating polyphenol, in the presence of compounds with antioxidant properties. The values of the antioxidant capacity of the fruit juices are expressed as vitamin C equivalents/L. The chromatographic method is based on the generation of OH radicals via Fenton’s reaction in order to test the inhibition of their formation in the presence of antioxidant compounds by monitoring salicylate aromatic hydroxylation derivatives as markers of •OH production, by means of HPLC coupled to coulometric detection. The results are expressed as the percentage of inhibition of •OH production in the presence of the tested juice compared to the control sample. When OH radicals are produced by Fenton’s reaction, the antioxidant capacity of the juices, estimated by both methods, displays an analogous trend, confirming that they can be considered an alternative for measuring the ability of antioxidants to block OH radical formation

Evaluation rubrics: a tool for laboratory activities assessment in chemistry and physics

Rubrics are a powerful tool for the evaluation of complex activities or authentic tasks. In these situations the final score is best represented when the task is subdivided in well defined portions, each of them describing the elements concurring to the overall performance. In this communication a set of evaluation rubrics for the assessment of several laboratory activities is presented, together with a rubric for the evaluation of the final report of an experimental work. This latter rubric is centered on the problem/hypothesis couple: the students are firstly encouraged to formulate a \u201cproblem\u201d, arising from the observation of a physical phenomenon. After the \u201cproblem\u201d has been defined, the students must find out an interpretative \u201chypothesis\u201d to be tested by the experimental process. Such an approach agrees with the indication of the Rocard Report on Education Science to increase interest and attainments in science. The introduction to the use of rubrics was presented to the students by means of a cooperative activity, centered on the examination of laboratory reports of different qualitative levels made by students of previous course years. The described strategy has been practiced both at university and in a secondary school (ITIS). A first approach has been also attempted within the \u201cPhysics and Chemistry Laboratory\u201d at LST. The extensive use of report rubrics for the whole period of the courses has increased both the understanding of how a scientific report should be organized and the student ability in effective written skills