Eden ReidJuly 28, 20203 min read

# Importance of Viscosity: How to Measure it

In simplest terms, viscosity is defined as “the resistance to flow” and is often referred to as the thickness of a fluid. The concept of a fluid having a “thickness” has existed for thousands of years, however the term “viscosity” was not introduced until 1929. Even Newton referred to viscosity as “the lack of slipperiness of the parts of the liquid” (Sir Isaac Newton, Principia, 1687).

When you think of a viscometer, what primarily comes to mind is an instrument capable of measuring viscosity. However, we know viscosity can be calculated, but cannot be measured directly. Viscosity can be calculated using various methods and there are numerous viscometers on the market today, but not all viscometers are made alike.

 Falling Ball Spindle Type Rotational Rectangular Slit Method Funnel Type Glass Capillary Oscillating Piston Stabinger Bubble Sample Size mL (some require 40 mL!) mL (6.7 - 16 mL) 2 - 20mL µl (26-100) Large mL mL (1.5 mL - 6 mL) mL (1.5 - 6 mL) mL Primary Measurement Time (Terminal Viscosity) Torque Stress Control or Shear Control Pressure Drop Time Time Shear Stress Shear Stress Time Viscosity Measured Dynamic Dynamic Dynamic Dynamic Kinematic Kinematic Kinematic Kinematic Kinematic Shear Rate Variation Yes (Must change the angle) Yes Yes Yes No No Yes No No Laws/ Principle applied Stoke’s Law, Hoeppler Principle Couette/Searle Couette/Searle Hagen-Poiseuille Flow Time Gravimetric Flow Electromagnetic Couette Time Benefits "Repeatable" Wide variety of sample capability ASTM standards Accurate characterization of shear rates Flexible to all types of geometry Characterizes viscoelastic behavior (G',G'')Thixotropy Small sample volumes Accurate measurements Easy to use Low reynolds numbers Quick quality control Simple design ASTM Standards Measures kinematic and dynamic viscosity also density Quick quality control Limitations Rely on Gravity-driven flow Shear rates not controlled or changed Difficult to use and clean No Viscoelastic characterization Hard to reproduce measurements Evaporation Irreversible adsorption of protein molecules at the interface Calibration before every experiment is key Careful attention to cleaning and fibers No Viscoelastic characterization Rely on Gravity-driven flow Shear rates not controlled or changed No Viscoelastic characterization End user variation easy to break and lose track of measurement No Viscoelastic characterization Variation from end users No Viscoelastic characterization No Viscoelastic characterization No Viscoelastic characterization End user dependent

When measuring viscosity, it is important to know if the fluid you are working with is Newtonian or non-Newtonian. Viscosity of Newtonian fluids is constant and therefore independent of shear rate whereas the viscosity of non-Newtonian fluids is not constant and therefore dependent on shear rate. (1) Many viscometers on the market today are only capable of accurately measuring viscosity of Newtonian fluids, when in reality most fluids are non-Newtonian. And, often only approximate the apparent properties of fluids, failing to quantify the absolute or true viscosity, which is one of the most important parameters in the development and modeling of applications that involve fluid flow. Behavior of Non-Newtonian fluids affects both production and how they are applied (ex: injectability, lubrication, spreading, jetting or drinking). Viscometers that don’t allow you to control shear stress or shear rate cannot adequately characterize Non-Newtonian liquids, and it is difficult to understand your sample’s behavior under production or application conditions without the ability to test under these same conditions.

 Common Newtonian fluids:(linear relationship between shear stress and shear rate) Common non-Newtonian fluids:(viscosity is not independent of shear rate) Water Mineral oils Low concentration protein Benzene Ethanol Shear Thinning Polymer solutions mAb solutions Paints Shear Thickening Corn starch solution wet sand Silica suspension in PEG Thixotropic Yogurt Gel Clay Colloids Bingham Pseudoplastic Ketchup Mayonnaise Lotion

RheoSense viscometers are powered by our patented VROC® (Viscometer-Rheometer-on-a-Chip) technology. VROC combines microfluidic and MEMS (Micro-Electro-Mechanical Systems) technologies to measure dynamic viscosity over a wide dynamic range of operation, delivering data with the highest accuracy and repeatability. Compared to conventional viscometers and rheometers, RheoSense’s rectangular slit method viscometers (USP chapter 914) offer several advantages:

• Small sample size requirement (26µL – 100µL)
• Cost-effective
• Characterization of both Newtonian & non-Newtonian fluids
• Enable high shear rates without flow instabilities
• Prevent evaporation and contamination of samples
• Render high throughput due to a simple flow-through design