Transient behavior of an electrorheological fluid in shear flow mode

Electrorheological (ER) fluids made of starch particles in silicone fluid were studied under different electric field strength, particle concentration, water content, and shear rate. The ER fluids were sheared under constant shear rates during at least 210 min and the shear stress was measured. According to the results, the shear stress increased with time until a maximum was reached, decreased a little, and then stabilized. Some samples did not show a point of maximum, but after increasing with time, they stabilized at a given shear stress. The influence of the shear rate on the shear stress depended on the time the sample was sheared, on the electric field strength, on concentration, and on water content. The results were analyzed in terms of changes of lamellar formations in the direction of shear. (C) 2000 The Society of Rheology. [S0148-6055(00)01105-6].4451139114

Superação da dormência em sementes de Bromelia balansae (Bromeliaceae) Overcoming seed dormancy of Bromelia balansae (Bromeliaceae)

Os efeitos da temperatura, substratos e tratamentos pré-germinativos para a superação da dormência foram avaliados na germinação de sementes de Bromelia balansae (caraguatá) oriundas de frutos coletados em Chapada dos Guimarães-MT. O delineamento experimental foi inteiramente casualizado com cinco repetições de 50 sementes por parcela. No primeiro experimento as sementes foram submetidas a seis tratamentos: a) 25°C e sobre papel, b) 30°C e entre papel, c) 25°C sobre papel, d) 30°C e entre papel, e) alternada 25/30°C e sobre papel e f) alternada 25/30°C e entre papel. No segundo foram avaliados os pré-tratamentos: a) ácido sulfúrico PA (98%) por 60 minutos; b) ácido sulfúrico PA (98%) por 15 minutos; c) ácido sulfúrico PA (98%) por 5 minutos; d) escarificação mecânica com lixa nº 80 e e) controle. No terceiro experimento os pré-tratamentos foram: a) ácido sulfúrico PA (98%) por 1 minuto; b) ácido sulfúrico PA (98%) por 3 minutos; c) ácido sulfúrico PA (98%) por 5 minutos; d) escarificação com lixa nº 80; e) imersão em água a 100ºC por 15 min. e f) controle. Nos dois primeiros experimentos houve baixa percentagem de germinação, com maior média de 25%. No terceiro experimento a germinação foi maior nos tratamentos com ácido sulfúrico (40% a 70%). As sementes de B. balansae apresentam dormência tegumentar que é superada com a imersão em ácido sulfúrico PA (98%) por 1 minuto.<br>The effects of temperature and substrates and different methods to overcome dormancy of Bromelia balansae (caraguatá) seeds were evaluated. The fruits were collected on Chapada dos Guimarães, Mato Grosso State, Brazil. The experimental design was completely randomized with five replications of 50 seeds for each sample. On the first experiment the seeds were submitted to six treatments: a) 25°C and over the paper, b) 30°C and over the paper, c) 25°C and between papers, d) 35°C and between papers, e) alternating 25/30°C and over the paper f) alternating 25/30°C and between papers. On the second experiment we evaluated a) immersion in concentrated sulfuric acid PA (98%) during 60 minutes, b) immersion in concentrated sulfuric acid PA (98%) during 15 minutes, c) immersion in concentrated sulfuric acid PA (98%) during 5 minutes, d) mechanical scarification with nº 80 sandpaper, e) control. On the third experiment we evaluated a) sulfuric acid PA (98%) for 1 minute; b) sulfuric acid PA (98%) for 3 minutes; c) sulfuric acid PA (98%) for 5 minutes; d) mechanical scarification with nº 80 sandpaper, e) immersion in water at 100ºC and f) control. On the two first experiments the seed germination was low. On the third experiment the seed germination was higher in the treatments with sulfuric acid (40% to 70%). The seeds of B. balansae present seedcoat dormancy that is overcome with the sulfuric acid immersion 100% for 1 minute