What Is Viscosity?

Fluid mechanics is the study of how fluids move, and one of the most important parameters of fluids is their viscosity. Most people think of “viscosity” in terms of a fluid’s ability to flow. Chemists have a slightly different feel for the phenomenon; viscosity is defined as its resistance to gradual deformation. This corresponds to the informal concept of “thickness” — for example, honey is thicker and has a much higher viscosity than water. Cleaning fluid viscocity plays an important role in how effective a cleaning fluid is. 

According to Wikipedia, viscosity is the friction between the molecules of fluid. For example, when a fluid flows through a tube, the fluid will move more quickly near the center of the tube axis but move more slowly along the walls of the tube. This is because of the friction with the walls. Pressure or energy (“stress”) is needed to overcome the friction between the faster-moving fluid and the more slowly moving layers. Fluids with low viscosity have a low resistance and shear easily and the molecules flow quickly; high viscosity fluids move sluggishly and resist deformation. Some liquids, like pitch, glass and peanut butter, have such high viscosity they behave like solids.

In common vernacular, a liquid is said to be “viscous” if its viscosity is substantially greater than that of water, and is described as “mobile” if the viscosity is noticeably less than water. Viscosity varies with temperature and pressure. It is usually expressed, particularly in ASTM standards, as centipoise (cPs). For example, water at 20°C has a viscosity of 1.002 cPs.

When it comes to parts cleaning, low viscosity is a plus, especially in densely packed PCBs or small parts with complex shapes. Low viscosity cleaning fluids pentrate into these tight spaces, clean thoroughly and then easily evaporate, or dissipate without leaving contamination or residue behind. Here are some MicroCare cleaning fluids that are known for their low viscosity trait of 0.42 cPs or less.

Tergo™ HDF Heavy Degreasing Fluid
Tergo™ XCF1 Cleaning Agent, Carrier Fluid & Particle Displacer
Tergo™ XCF2 Specialty Cleaning Fluid, Flux Remover and Degreaser
Tergo™ XCF3 Superior Cleaning Fluid and Degreaser
Tergo™ CCA Broad Range Degreasing Fluid
Opteon™ SF80 Specialty Cleaning Fluid

Some viscosities for other common fluids are listed in this chart for comparison:

Fluid Viscosity (cPs)
Air  0.02
Acetone  0.3
Methanol  0.6
Water  1.0
Ethanol  1.2
Mercury  1.5
IPA 2.40
Olive oil  84.0
Motor Oil SAE 50  540.0
Maple Syrup   3,200.0
Honey  12,200.0
Molasses (treacle)  20,000.0
Peanut Butter  250,000.0
 

 

Cold molasses is a highly viscous liquid, as every grandmother knows.
Cold molasses is a highly viscous liquid, as every grandmother knows.