Viscosity and Rheology for the perplexed
Viscosity
Viscosity is a measure of the resistance of a fluid. Honey has a high viscosity value and water has a lower viscosity. Bourbon has a lower viscosity than water.
Next time you are at an SMTA (or worse IEEE) dinner, listening to some assembly or application conversations and feeling a little inferior to all those PhDs and Wantabe PhDs making you feel like a loser. This is your “word”. “Rheology”. There is no undergrad course in this. There are few courses at the graduate level. However, it is of key importance to the industry and the world. Put your chin up high and say, “if you consider the role of Rheology in electronic and solar assembly you would see how this issue can be addressed…”
Rheology
The word Rheology comes from rheo, from the Greek word for flow, and –ology, meaning study. It is about how different materials (fluids) flow, change shape and retain shapes. It is about how a fluid reacts inside a valve or pump and how it flows out of the nozzle, and then how it flows on the board.
Out of a thousand people, 2 or 3 will have a murky understanding of this (Oh, feeling a little superior now are we?). A clear understanding is often found in a couple of material scientist inside material manufacturing plates and further subjugated to the R&D departments and mostly on the “R” side. These guys are normally the introverted egg heads that will instantly and immensely respect you for what you will be able to explain by reading the following:
Fluids come in a wide range of behaviors. The best analogies and examples commonly related to (and conveniently availably at the dinner) are foods.
Types of fluids
Newtonian – A Newtonian fluids (named after sir Isaac Newton) are any fluid whose stress versus strain rate curve is linear and starts at zero. In other words in a valve, a light valve actuation pushed this fluid, and heavier actuations push it more. How much more is a linear function. The first graph shows this
It helps the story if you are stirring your Bourbon with the straw when you describe this.
Non-Newtonian fluids
Non-Newtonian fluids
This leads us to the Non-Newtonian fluids (now take a sip of the Bourbon)
Pseudo plastics and shear thinning. You know the behavior of these types of materials and don’t even know it. Condensed milk, orange juice, Ketchup, mayonnaise, peanut butter are examples of pseudo plastic foods. The quicker you spread them, the easier it flows. This is classic shear thinning. These are also the kind of materials used in dispensing. They are easy to force out under pressure and stay where you put them. Another way to imagine what is happening is to see the particles are little plate like structures. When pushed, the plates align and slip against each other (flows) and at rest they are random (a stiff structure). These are slippery under stress.
Dilatants and Shear Thickening. Examples are a little harder to find, Dilatant fluids are tightly packed particles with a small amount of liquid filling the gaps between the particles. This fluid lubricates the mixture and flows easily. If the liquid filling the gaps is stiffer, there is a lot of friction or resistance. And the viscosity increases. Examples include cornstarch and water, liquid Chocolate and Silly Putty
Visco-Plastic fluids
One step more and you will have an understanding of Rheology for dispensing. (Ask for the bill and take another sip of your drink – it is nearly time to leave) Some materials resist for a while and then break lose. They have a “Casson Value” or a Yield Point. These two terms are nearly identical. Ketchup is a good example – a small tap – nothing moves, a hard tap and boom… it is on your shirt. The amount of the “tap” is the yield point.
About now you should have blown away the SMTA or IEEE dinner guests, but shortly the “mathematician” is going to ask for the equations. When the bill arrives, pay it and the tip, down the late of the bourbon and bid the crowd “adu” and leave, you have just demonstrated to the room you have a working and practical knowledge of Rheology. No need to wait for questions – head for the door