Viscosity

Viscosity represents a fluid’s resistance to flow – essentially, how strongly it resists deformation when a force is applied. It’s a measure of the internal friction within the fluid, describing its ability to impede the movement of adjacent layers. Fluids with high viscosity, such as honey or molasses, exhibit a greater resistance to flow compared to fluids with low viscosity, like water or alcohol. This resistance stems from the cohesive forces between the fluid molecules. Understanding viscosity is critical in various applications, including fluid dynamics, lubrication, and the formulation of paints and adhesives.

Units and Measurement

• The SI unit of viscosity is the pascal-second (Pa·s).
• A more commonly used unit in labs is the centipoise (cP), where 1 cP = 0.001 Pa·s.
• Viscosity is measured using instruments like viscometers and rheometers.

Temperature Dependence

Viscosity is highly sensitive to temperature:

• For liquids, viscosity decreases as temperature increases (they flow more easily).
• For gases, viscosity increases with temperature due to faster molecular motion.

This behavior is crucial in applications like lubrication, chemical processing, and biological systems.

Molecular Basis

Viscosity arises from intermolecular forces:

• Stronger forces (e.g., hydrogen bonding in glycerol) lead to higher viscosity.
• Weaker forces (e.g., van der Waals in hexane) result in lower viscosity.

The shape and size of molecules also matter—long, entangled molecules (like polymers) tend to have higher viscosity.
 

Applications

• Chemical engineering: Designing reactors and pipelines
• Pharmaceuticals: Controlling drug delivery and formulation
• Food science: Texture and mouthfeel of sauces and syrups
• Cosmetics: Cream consistency and spreadability
• Biology: Blood viscosity affects circulation and health

Products Ranked by Viscosity: From Fluid to Thick

Low Viscosity (Flows Easily)

• Water – baseline fluid with very low viscosity (~1 cP)
• Ethanol – used in disinfectants and perfumes (~1.2 cP)
• Acetone – common solvent in nail polish remover (~0.3 cP)
• Gasoline – highly fluid and volatile
• Paint thinners – designed to reduce paint thickness

Medium Viscosity (Moderately Thick)

• Olive oil – flows slowly, used in cooking (~80 cP)
• Motor oil (SAE 10W-30) – used in engines (~100–150 cP at room temperature)
• Milk – slightly thicker than water (~2–3 cP)
• Maple syrup – sweet and sticky (~200–300 cP)

High Viscosity (Thick and Slow-Moving)

• Honey – very thick, especially when cold (~2,000–10,000 cP)
• Glycerin – used in cosmetics and pharmaceuticals (~1,000 cP)
• Toothpaste – holds its shape, doesn’t flow (~70,000–100,000 cP)
• Silicone sealant – used in construction, extremely viscous
• Butter or margarine (at room temperature) – semi-solid, barely flows