TECHNICAL NOTE

Understanding effects of viscosity in the BioFlux system

Viscosity

Introduction

In the BioFlux software, the user inputs the desired shear stress under which to run the experiment. From this input value, there are equations used to populate the other fields visible to the user. These equations calculate values for shear stress, volumetric flow rate, run time and pressure. In order to apply the correct amount of air pressure to the plate, and thus apply the correct shear, the equations must factor in the correct viscosity of the fluid. The viscosity of a fluid describes its resistance to flow. In the BioFlux software, this value is entered in the Fluid Properties manager. A number of commonly used fluids are preset in the software, such as water and PBS. Custom fluids (and their viscosities) can also be entered in the Fluid Properties module. This technical note will describe some of the important aspects of viscosity and serve as a guide for planning experiments in the BioFlux system.

Relationship Between Temperature and Viscosity

Shown below (Figure 1) is a graph of viscosity versus temperature for water. As the temperature increases the viscosity decreases. Note that at lower temperatures (10-40˚C) there is a linear relationship and at higher temperatures the slope changes. For exact values, the viscosity of a given fluid can be measured in the laboratory with a viscometer. We used a Cannon-Fenske Routine Viscometer, size 50 (VWR catalog #52946-777) to measure the viscosity of several commonly used fluids at room temperature and 37˚C. It is accurate to ± 0.17%. As you can see, the viscosity drops with the increase in temperature for all fluids measured (Figure 2).

Figure 2: Viscosity was measured for 6 commonly used fluids at room temperature and 37˚C. Viscosity drops with an increase in temperature for all fluids.

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Figure 1: A plot of viscosity versus temperature for water. Viscosity change is approximately linear from 10-40˚C but the slope changes when the temperature rises above 40˚C.

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y = -0.0217x + 1.483R² = 0.9731

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TECHNICAL NOTE

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ViscosityUnderstanding effects of viscosity in the BioFlux system

How this impacts flow conditions in the BioFlux system

Prior to starting an experiment, make sure you have your fluid entered in the Fluid Properties manager. Consider if you will be running the experiment at room temperature or at an elevated temperature (i.e. 37˚C). If your fluid is not one of the preset values, it is always a good idea to measure the viscosity of your fluid empirically using a viscometer. You can also make reasonable assumptions by doing linear extrapolations from known values published here or in the literature. It is worth noting that seemingly small changes in viscosity can have large effects of the fluid dynamics. For example, PBS has viscosity equal to 1.05cP at room temperature, but drops to 0.70cP at physiological temperature. This represents a 33% increase in flow rate to keep the same shear stress. Having the correct viscosity entered in the software has two main benefits: 1) it will provide a more accurate representation of the shear stress profile, and 2) it will help make more accurate predictions over run time per well (useful for longer experiments that may require refilling of the wells).

The molecular composition of the fluid also affects the viscosity

The interactions among molecules comprising a particular fluid contribute to the overall viscosity of the fluid. The nature of these interactions contributing to the viscosity is complex. As a general rule, most saline-type buffers we have measured have approximately the same properties and the same viscosities as expected by their compositional similarity. However, when measuring media with increasing amounts of solutes, an increase in viscosity is observed. This is the case with rich bacteriological growth medium (i.e. TSB) and mammalian tissue culture medium (William’s Complete) with fetal bovine serum added which both contain large amounts of proteins.

A note about blood viscosity

Blood viscosity can be considered a special case. Blood viscosity depends on temperature and red blood cell concentration (hematocrit), which depends on the gender and health status of the donor. It is also dependent on the flow rate. At very low flow rates the cell and molecu-lar interactions increase to a pathological state enabling red blood cell adhesion. For most intents and purposes, average blood viscosity at 37˚C is around 3-4 cP. However, attention must be paid to the hematocrit. For example, at a hematocrit of 40 (40% volume red blood cells / total liquid volume) the viscosity is 4 cP, but increasing the hematocrit to 60% doubles the viscosity to 8cP. The effects of temperature should also be considered. For each degree C in temperature decrease there is a 2% increase in blood viscosity.

Further information

Kameneva MV (1999) Gender difference in rheologic properties of blood and risk of cardiovascular diseases. Clin Hemorheol Microcirc. 21(3-4):357-63.Viscosities for other common fluids can be found at:http://www.engineersedge.com/fluid_flow/fluid_data.htmhttp://www.cvphysiology.com/Hemodynamics/H011.htm, http://en.wikipedia.org/wiki/Viscosity

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19° 22° 23° 37°PBS-CMF 1.05 0.70HBSS 1.01 0.70M63 Minimal Salts 1.08 0.70LB 1.10 0.74TSB 1.10 0.88Williams complete+10%FBS 1.20 1.17 0.97

1 D-Phosphate buffered saline – CaMg free (UCSF CCFAL003) 2 HBSS with calcium and magnesium, without phenol red (Hank’s BSS) (UCSF CCFAJ002) 3 M63 Minimal Salts (Pardee et al 1959 + 1mM sodium citrate+ 1mM MgSO4) 4 Luria Broth - Teknova L8050 5 TSB – TBD BBL Trypticase soy broth (ref #299113) 6 Williams complete+10%FBS (Invitrogen cat #12551-032)