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Name: Rebecca
Status: student
Grade: 6-8
Location: GU



Question:
How does the viscosity of a fluid change with pour speed?


Replies:
Here is a YouTube link that shows the corn-starch water 'shear-thickening' behavior: http://www.youtube.com/watch?v=f2XQ97XHjVw If you search 'non-newtonian fluid' you can find a lot more information. Burr Zimmerman


The fluid mechanics of "Pouring" is very complicated. I suspect that there is no comprehensive quantitative model describing this very "elementary" every day process. A couple of examples: BREAKPOINT -- If you pour a liquid from a non-circular orifice, the stream is not circular. The cross section alternates between being "prolate" / "oblate" / "prolate" / "oblate". Then at some distance from the orifice the stream "breaks up" forming droplets. This process is was first studied by Lord Kelvin in the 19th century, and continues to be an area of active experimental and theoretical interest. Male readers can view this type of flow every time they urinate! ENTANGLEMENT -- If the fluid viscosity results from entanglement of long polymer chains, two changes in the flow properties result. First, there is the buildup of a "ball" at the lip of the container. This results because the polymer chains disentangle slowly compared to the gravitational force that is causing the fluid to flow. So the molecules in the vessel tend to hold on to those molecules that are beginning to flow under the force of gravity. On the other side, once flow has begun it is difficult to stop because the same entanglement tends to keep pulling more fluid. You can see this behavior in syrup or honey.

I have only mentioned two of many characteristics that influence the behavior of liquid under "pouring" conditions.

It is a very complicated phenomenon to analyze.

Vince Calder


Hi Rebecca,

This is an insightful question that has several answers. Most common fluids, such as water, most oils, gasoline, and so on, are called "Newtonian Fluids", which means their viscosity depends only on temperature and pressure, and not the forces acting on them (such as turbulence caused by fluid motion). For these common fluids, the answer to your question is that the viscosity is entirely unaffected by fluid flow (or pour speed as call it); viscosity does not change at all.

There is a second class of fluids called Non-Newtonian Fluids that act differently. With these fluids, stress on the fluid (caused by flow, or pushing it through a small orifice, for example) does result in an apparent viscosity change. A common example of this is ketchup. A good bottle of ketchup can be turned upside down, and is so viscous that it will not flow much (or at all). But if you stir or shake it to cause the ketchup to begin to flow, this small increase in flow causes its viscosity to reduce, resulting in even lower viscosity and faster flow. Some other types of Non-Newtonian Fluids do the reverse; increased flow causes their viscosity to increase, not decrease.

So to summarize, for most common ("Newtonian") fluids, flow rate will not affect viscosity at all. However some less common ones called "Non-Newtonian" fluids, do undergo an increased or decreased viscosity change with increased flow. The actual change in viscosity for these fluids varies from one type to another, and of course also varies depending on flow rate, so is not easily predictable.

Regards,
Bob Wilson.


Rebecca,

The complete answer to this question is very complex and has many variables. However, if we limit ourselves to some very general statements, we can narrow down the answer to some basic ideas. First of all, we need to define some of the important factors that contribute to viscosity. Viscosity, as you know, is the property of a fluid to "hang on" to itself as it flows. Factors that contribute to this property are (to name a few): intermolecular attractive forces (which contribute to density and interaction with the walls of the container), temperature, and physical interaction (such as molecular entanglement, molecular shapes, molecular size). Second, we need to understand that the shape of the container will have an impact on the flow of the fluid. Bends in the tube, roughness of the tube surface, cross-section of the tube, type of material and its chemical interaction with the fluid - all have an effect on the viscosity. Third, external forces applied to the fluid may cause the fluid to react in a non-linear way. For example, while most fluids will speed up its flow when a force is applied to it, other fluids may actually resist the flow when more force is applied (and worse, this resistance varies depending on the strength and speed of the force applied).

So, if we limit ourselves to the simplest of fluids (such as water and most gases), and we exclude any kind of chemical or physical interaction of the fluid with the walls of the container, then we have a system that is called "newtonian". For newtonian fluids, viscosity is defined as the force required to make a fluid flow versus how much actual flow there is. This viscosity is a constant and is dependent only on temperature and pressure. Thus, we do not expect newtonian fluids to have varying resistance to flow (change viscosities) as a function of flow or force. Water, for example, continues to flow at the same rate whether we stir it fast or slow.

Unfortunately, very few liquids are newtonian. And this is why the complete answer is complex because all the other factors we eliminated to get to a newtonian fluid now apply. You may have observed that honey flows faster when we the only force applied to it is gravity. Trying to force honey out of a squeazable bottle sometimes (but not always) results in the honey flowing out even slower.

Greg (Roberto Gregorius)


For many fluids, the viscosity is pretty constant. Fluids that have constant viscosity are called 'Newtonian' fluids. Common fluids like water, solvents you would see in a chemistry lab, etc. are Newtonian - meaning their viscosity does not change. When you start mixing in things -- especially large molecules like starches or proteins (like you would find in things you might drink), things no longer act "normally".

Fluids whose viscosity changes, called "non-Newtonian" fluids, often contain large molecules (like polymers). One easy example is a mix of cornstarch and water, which gets thicker the faster you pour it. In industry, polymer melts (the hot liquid plastic used for making plastic parts) are typically non-Newtonian (cornstarch and plastic are both made of very large molecules). Their viscosity may go up or down depending on how fast you pour them.

Some liquids lower their viscosity when pouring (examples: blood, milk). Others get thicker (cornstarch in water). Yet others have very complex relationships -- for instance ketchup, honey, and some paints change viscosity based on how *long* you apply a shear stress.

In scientific terms, we talk about how the viscosity changes with respect to 'shear' -- shear is a sideways force on the fluid. When you change the rate you pour something, you change the shear force. Actually, depending on how you pour, you may or may not be proportionally changing the shear on the fluid. A better experiment might be to put a thin layer of fluid between two flat plates, and measure the force required to rotate the top plate at a given speed. This is easy to make and gives very good data. Or, you could buy a 'viscometer' -- although this may be expensive.

I hope this is helpful,
Burr Zimmerman



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