Surface and Colloid Chemistry Rheology

Surface and Colloid Chemistry Rheology

• Rheology is the study of flow
• It's a Greek word, combining "rheo" (flow) and "logia" (study)
• It examines how substances deform and flow, especially those exhibiting time-dependent responses to stress
• Introduced by Professor Eugene Cook Bingham
• Bingham plastic fluid is named after him
• Based on principles of elasticity (Hooke, 1678) and viscosity (Newton, 1687)
• Fluids are made of overlapping layers
• Flow of liquids is analogous to a stack of cards slipping over each other
• Shear stress is the force applied to a liquid area
• Shear rate is the speed gradient of liquid layers
• Shear stress (τ) and shear rate (γ) have a ratio, η (viscosity) in Newtonian fluids, τ = ηγ
• Viscosity is a measure of resistance to internal friction or flow
• Internal friction is caused by forces between molecules at interfaces
• SI unit is Pascal.sec (Pa.S), CGS unit is poise (P)
• 1 mPa.s = 1 cP
• Hooke's elasticity and Newton's viscosity laws are linear
• Viscoelasticity can make the linearity of the relationship between shear stress and shear rate change over time

Newtonian Fluids

• Constant ratio between shear stress (τ) and shear rate (γ), η (viscosity)
• η = τ / γ
• Viscosity doesn't change with shear rate
• Tension in the simple shear flow is zero where the two normal tensions differences are zero
• Viscosity is constant during shearing
• Viscosity returns to zero when shearing stops
• Viscosity values at different deformations are proportional

Non-Newtonian Fluids

• Deviate from Newtonian flow behavior
• Linear relationship between shear stress and shear rate is impaired
• Examples include: colloidal solutions, emulsions, ointments, clay dispersions

Flow Types

• Dilatant Flow: Viscosity increases with increasing shear rate or time

  • Literature terms: "condensed flow" or "shear thickening flow"
  • Examples: cornstarch, vinyl resin pastes, sand-water mixtures, 50% solid content suspensions

• Pseudoplastic Flow: Viscosity decreases with increasing shear rate

  • Also called "shear thinning"
  • Examples: Paints, emulsions, polymeric solutions. This is the most common type of non-Newtonian flow

• Bingham Plastic Flow: Exhibits solid-like behavior under stable conditions

  • Requires a specific yield stress for flow to start
  • Examples: Flocculated suspensions (1-10% solid content), mayonnaise, ointments, toothpaste, ketchup

Additional information

• Summary/lab compensations will be covered in the upcoming week.

This quiz covers the fundamentals of rheology, the study of flow in substances, focusing on key concepts such as shear stress, shear rate, and viscosity. It also highlights the contributions of notable figures like Professor Eugene Cook Bingham. Test your understanding of how materials deform and flow under stress.