Wonderful Lectures! I am really enjoying them. I would have a question, if I may: I received a graph (shear-rate in x vs shear-stress in y) from a lab (related to a body lotion bulk). This graph looks like power-law type and the shear stress is increasing with shear-rate until a certain value of shear-rate (nothing extremely high: I would say until 300 sec^(-1) ). Then something strange happens: it starts to decrease with shear-rate increasing starting from that point. Is that anything logic? It sounds really strange to me by physical point of view: to increase shear rate I need to reduce the shear-stress! Am I interpreting it wrongly. I know they needed to use two different viscosimeters in the two intervals of shear-rae and I am wondering if they just made some mistakes in "welding" the two curves. Thank you so much in advance for your insights and I hope my english is clear enough!
Thank you for your feedback - and I'm glad you're enjoying the course material. There's a number of things that could be happening in the data you describe - which it is is hard to say without seeing the data and understanding exactly what experiments were done to get the data. If two different data sets have been superposed to get the data you're looking at, it's perfectly plausible that one of the data sets is just wrong - it's very easy to get bad data from a rheometer if you're not careful! If the data is correct, though, then you could be looking at a number of possible phenomena - if you find that a plot of apparent viscosity is more-or-less constant as a function of increasing shear rate and then suddenly drops (which your stress vs rate plots may infer), then *either* you've got the onset of yielded with a material that's viscoplastic *or* you've got some shear thinning happening but no plasticity. Both of these have a similar underlying cause, which is a structural change in the fluid brought about by increasing shear. One of these structural changes causes the material to flow (yielding) and the other is a more subtle structural change in an already flowing fluid that manifests as a reduction in shear viscosity. But without knowledge of the data and of the experiments it's hard to say which of these options you've got! Hope that helps.
@@drbartsworldofchemeng Thank you very much for your quick answer. It looks like they used one only instrument. Is there any way I can send you more info (excel file and the used rheometer)? this is intriguing me a lot since i cannot understand the underlaying "mechanics": if the spindle has a certain speed (in the decreasing region of shear-stress), it is like saying that in order to increase the speed, I need to reduce the torque (and so the shear stress)! Thanks a lot again for your insightful comments
@@marcozavarella9352 Hi Marco, if you go to the PayHip site that my design book is for sale on, you can send me a message there and then we can chat via email.
Once again a very nice lecture!~
At 7:30 I think the right part of the u(rc) should also be divided by μΒ?
Wonderful Lectures! I am really enjoying them. I would have a question, if I may: I received a graph (shear-rate in x vs shear-stress in y) from a lab (related to a body lotion bulk). This graph looks like power-law type and the shear stress is increasing with shear-rate until a certain value of shear-rate (nothing extremely high: I would say until 300 sec^(-1) ). Then something strange happens: it starts to decrease with shear-rate increasing starting from that point. Is that anything logic? It sounds really strange to me by physical point of view: to increase shear rate I need to reduce the shear-stress! Am I interpreting it wrongly. I know they needed to use two different viscosimeters in the two intervals of shear-rae and I am wondering if they just made some mistakes in "welding" the two curves. Thank you so much in advance for your insights and I hope my english is clear enough!
Thank you for your feedback - and I'm glad you're enjoying the course material. There's a number of things that could be happening in the data you describe - which it is is hard to say without seeing the data and understanding exactly what experiments were done to get the data. If two different data sets have been superposed to get the data you're looking at, it's perfectly plausible that one of the data sets is just wrong - it's very easy to get bad data from a rheometer if you're not careful! If the data is correct, though, then you could be looking at a number of possible phenomena - if you find that a plot of apparent viscosity is more-or-less constant as a function of increasing shear rate and then suddenly drops (which your stress vs rate plots may infer), then *either* you've got the onset of yielded with a material that's viscoplastic *or* you've got some shear thinning happening but no plasticity. Both of these have a similar underlying cause, which is a structural change in the fluid brought about by increasing shear. One of these structural changes causes the material to flow (yielding) and the other is a more subtle structural change in an already flowing fluid that manifests as a reduction in shear viscosity. But without knowledge of the data and of the experiments it's hard to say which of these options you've got! Hope that helps.
@@drbartsworldofchemeng Thank you very much for your quick answer. It looks like they used one only instrument. Is there any way I can send you more info (excel file and the used rheometer)? this is intriguing me a lot since i cannot understand the underlaying "mechanics": if the spindle has a certain speed (in the decreasing region of shear-stress), it is like saying that in order to increase the speed, I need to reduce the torque (and so the shear stress)!
Thanks a lot again for your insightful comments
@@marcozavarella9352 Hi Marco, if you go to the PayHip site that my design book is for sale on, you can send me a message there and then we can chat via email.
good
Thank you!