Viscosity and Pour Speed

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

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