No abstract

In this work, we will describe an experimental setup for a standing-wave ultrasound trap for air microbubbles in oil. We develop a model for the finite acoustic beam using the angular spectrum technique, and reconstruct the pressure field using the General Lorenz-Mie Theory framework, which was validated using a finite elements method (FEM) simulation. Using Stokes' drag law, we were able to obtain the radius of the trapped bubbles and estimate the minimum acoustic force necessary to trap them, which ranged from 3 nN to 780 nN. We also present the force profile as a function of distance for different bubble that were trapped experimentally, and show that a standing wave formed by interfering infinite plane waves cannot explain the observed acoustic trapping of bubbles in 3-D.

We built a magnetic stirrer to mix tea and honey and developed a method to determine the homogeneity of the mixture based on the variation of light intensity of a laser beam with time due to its scattering by the mixture. The addressed problem was that of mixing tea and honey as fast as possible with a power limitation of one single 1.5 V battery, following the statement of Problem 15 of the 9th International Physicists’ Tournament. The influence of temperature and honey concentration on the total time of mixing was studied through the performance of several experiments reported here. In the best configuration of parameters, the total mixture occurs in less than 2 s, while in the worst configuration, the time necessary reaches 10 s. Results validate the magnetic stirrer as a proper device for mixing tea with honey and light attenuation as an excellent physical property for investigating the homogeneity of the mixture of the honey in tea. Supplementary investigations were made exploring the influence of adding sugar and or a slice of lemon to the tea.

Neste artigo, discutimos a física envolvendo uma câmara de radiação ultravioleta–C (UV–C) utilizada para desinfecção de superfícies e objetos, no contexto da pandemia de COVID-19. Quando exposto à radiação UV–C, o RNA viral muda sua estrutura molecular num processo causado por uma realocação nas bases nitrogenadas, inativando o vírus e impedindo sua reprodução. Nós propomos a construção de uma câmara UV–C utilizando materiais acessíveis à população, com recursos de segurança. Também discutimos o processo de design na determinação da distância e tempo de exposição necessários para a dose de UV–C apropriada. Fatores teóricos que deixam o problema mais realista considerando-se a geometria do sistema também são utilizados. Experimentalmente, a lâmpada utilizada para o protótipo foi testada, obtendo-se uma potência na faixa do UV–C de Puv = 171μ W, após correções. Os tempos de exposição foram então definidos, e variaram de 20–90 minutos, considerando-se distâncias de 5–36 cm dos objetos à lâmpada.