Almost every day we have questions about the difference between viscosity, consistency and surface tension. Usually the problematic second part;
"How do we relate the measurements of these different properties?"
The simple answer is that they are different, they are measured differently, and the measurements cannot be correlated.
A common element is the property that each is a liquid material. Attempts to convert measurements between these properties lead to broken failure or convolution approximation algorithms.
However, each of these measurements has a significant impact on formulation development, production methods and final quality of many types of products. The importance of these properties of liquids in industries as diverse as healthcare, food production, coatings and pharmaceuticals cannot be overemphasized.
Each of these liquid properties is clearly demonstrated and some examples of how these measurements are used in everyday products are shown.
definition
Viscosity is simply defined as a measure of a liquid's resistance to flow. This simple definition is complicated by the reality of actual liquids.
Some respond in a reasonable way and have the same resistance over a wide speed range.
Some increase drag with speed, while others decrease with speed
Some people change resistance over time.
This creates some interesting measurement challenges. A common feature for all viscosity values is that they vary with temperature.
Consistency is a property related to firmness or thickness.
This term is used for many materials that only exert gravity flow.
Observing the degree or distance of flow over a specified period of time is the rationale for these measurements.
Measured terms are usually derivations of time and distance and cannot be translated into standard rheological terms.
Like viscosity, consistency changes with temperature.
Surface tension is the easiest concept to define. It is the attraction between molecules on the surface of a liquid. This attraction keeps liquids from flying into the air and resists surface penetration.
When surface tension is lowered, liquids mix more easily with other materials. For example, detergents lower water surface tension, which allows water to penetrate clothing fibers and rinse away dirt.
Watch a spider walk on water and observe the effects of surface tension. Spider legs do not penetrate surfaces.
Surface tension is an essential feature in contact angle analysis. This is the interaction of a liquid with a solid surface or substrate.
This is another liquid phenomenon that is affected by temperature.
Each of these measurements, viscosity, consistency, and surface tension, have two things in common. Each is a property of the liquid, and each is affected by temperature.
Below are some examples of the use of these measurements in the development and production of common products.
Industry application
In the coatings industry, several product properties can be defined by these liquid properties.
The effect of surface tension is directly related to the adhesion properties. Adhesives are important in the development of inks and paints because they need to be designed to adhere to different surfaces.
Surface tension and associated contact angle analysis are key to the association of coatings and inks with solid surfaces or substrates.
The importance of viscosity level in developing coating application methods and systems cannot be overstated as this property affects flow characteristics.
Petroleum product specifications are usually defined in terms of viscosity.
An example would be motor oil names such as 10W/30 - Viscosity Reference.
Viscosity measurement and comparison is used to monitor the deterioration of lubricating oils.
Surface tension testing is used to detect impurities in products such as transformer oil.
Food production has many applications for these liquid properties.
Surface tension is an important factor in determining mixability characteristics, such as the choice of different liquid components for a salad dressing.
The value of viscosity measurement lies in the development of products such as ketchup formulations to obtain a good combination of thickness and fluidity.
The processing quality control standard for many tomato products, dressings and condiments such as mustard is a consistency measuring device called a Bostwick Consistometer.
In the pharmaceutical world, surface tension is an important element in the potency or design of drug delivery systems. For example:
Develop effective binder relationships, such as polymers or starches, to drug particles in capsules
Development of complex liquid mechanical structures to define release conditions for novel liposome-focused drug delivery systems
Special delivery requirements such as eye drops.
These development activities translate into materials and production control applications where viscosity measurement is often a quality standard.
Waterproofing is an example where product development relies heavily on surface tension.
Products used in waterproof tent fabrics are based on the relationship of the spatial dimensions in the weave of the fabric to the adhesion or surface tension of raindrops.
For those used to camping, these fabric tents will turn on the rain.
But if you press your finger on the droplet, it pushes it through a small open space in the fabric.
Blood viscosity may be a single parameter associated with cardiac risk factors such as:
High blood pressure, high LDL, type II diabetes and obesity
High blood viscosity is the primary mechanism by which these factors transmit major inflammatory damage to the arterial wall.
Blood viscosity is usually measured using an automated scanning viscometer based on capillary technology.
