Fluid Resistance to Flow
A property exhibited by fluids, encompassing both liquids and gases, characterizing their resistance to deformation by shear stress or tensile stress. It describes a fluid's internal friction or the opposition it presents to flowing. It is a critical parameter in fluid dynamics and various engineering applications.
Types of Fluid Behavior
- Newtonian Fluids: Fluids that exhibit a linear relationship between shear stress and shear rate. Their resistance remains constant regardless of the applied force or agitation. Examples include water, air, and mineral oil.
- Non-Newtonian Fluids: Fluids where the relationship between shear stress and shear rate is non-linear. Their resistance can change under stress. Examples include ketchup, blood, paint, and some polymers.
- Shear-Thinning (Pseudoplastic): The resistance decreases with increasing shear stress (e.g., paint).
- Shear-Thickening (Dilatant): The resistance increases with increasing shear stress (e.g., cornstarch suspension).
- Thixotropic: The resistance decreases with time under constant shear stress (e.g., yogurt).
- Rheopectic: The resistance increases with time under constant shear stress.
Factors Affecting Fluid Flow Resistance
- Temperature: Generally, increased temperature decreases the resistance of liquids while increasing that of gases.
- Pressure: Increased pressure typically increases the resistance of gases, especially at higher densities. The effect on liquids is usually minimal.
- Intermolecular Forces: Stronger intermolecular forces between fluid molecules lead to higher internal friction and resistance.
- Molecular Size and Shape: Larger and more complex molecules tend to create greater resistance.
- Fluid Composition: The chemical makeup of a fluid directly affects its intermolecular forces and structure, influencing its resistance.
Measurement Techniques
Viscometers and Rheometers
Instruments used to measure the resistance of fluids. Viscometers are typically used for Newtonian fluids, while rheometers are more versatile and can be used to characterize non-Newtonian behavior.
- Capillary Viscometers: Measure the time it takes for a fluid to flow through a capillary tube.
- Rotational Viscometers: Measure the torque required to rotate an object immersed in the fluid.
- Falling Ball Viscometers: Measure the time it takes for a ball to fall through the fluid.
Applications
Understanding and measuring the resistance of fluids is crucial in numerous fields:
- Engineering: Design of pipelines, pumps, and lubrication systems.
- Manufacturing: Control of the consistency and flow properties of paints, coatings, and polymers.
- Food Science: Determination of the texture and flow behavior of food products.
- Medicine: Study of blood flow and other physiological fluids.
- Geology: Analysis of magma flow and the behavior of earth materials.