Surface and Colloid Chemistry Rheology PDF

SURFACE and COLLOID CHEMISTRY Prof. Dr. Orhan OZDEMIR January 2025 ▪ What is Rheology? ▪ Rheology is a Greek Word. ▪ Rheo is «Flow» ▪ Logia is «Study» ▪ Rheology is a STUDY of FLOW. ▪ Rheology is the science that st...

SURFACE and COLLOID CHEMISTRY Prof. Dr. Orhan OZDEMIR January 2025 ▪ What is Rheology? ▪ Rheology is a Greek Word. ▪ Rheo is «Flow» ▪ Logia is «Study» ▪ Rheology is a STUDY of FLOW. ▪ Rheology is the science that studies the deformation and fluidity of a substance or materials that exhibit a time- dependent response to different stress conditions. ▪ It was first introduced by Professor Eugene Cook Bingham. ▪ The type of fluid known as a «Bingham plastic fluid» is named after him. ▪ Rheology is based on the principles of elasticity developed by Hooke for solids in 1678, ▪ and; ▪ the viscosity principles that Newton introduced in 1687. ▪ Newton thought that the fluids are composed of overlapping layers. ▪ Newton likened the flow of liquids to the movement of a stack of cards slipping over each other. A V, F y d F x ▪ The flow of liquid between two parallel layers with a distance d and A surface area between them is taken into account. ▪ Thus,  speed gradient is called «shear rate». ▪ Under the influence of the applied force F: 𝐹 𝜏= 𝐴 ▪ A shear stress results in flow according to Newton, ▪ There is a certain ratio between this «shear stress» 𝝉 (tau) and shear rate, 𝜸 (gamma), 𝜼 (eta) being a coefficient; 𝜏 = 𝜂. 𝛾 ▪ The coefficient 𝜼 here is called «shear viscosity» or simply «viscosity». ▪ Viscosity can be described as a measure of the resistance to an internal friction or flow. ▪ This friction is caused by the effective forces between the molecules at the interfaces between adjacent layers. Unit of viscosity; ▪ According to SI unit system: Pascal.sec (Pa.S) ▪ According to CGS unit system: poise (P). ▪ 1 mPa.s (milli pascal sec) is equal to 1 cP (centipoise). ▪ Hooke's elasticity and Newton's viscosity laws are linear expressions directly related to the ratio between tension and shear. ▪ After the concept of viscoelasticity was introduced, it was observed that this linearity may deteriorate or change the viscosity over time depending on the properties of the substance, depending on the applied shear stress, and shear rate. ▪ Therefore, fluids are mainly classified as: ▪ Newtonian fluids ▪ Non-Newtonian fluids Viscoelasticity ▪ Shear stress vs. shear rate graphs of Newtonian and Non- Newtonian flow types. BINGHAM PLASTIC Toothpaste (𝝉, 𝒌𝑵𝒎𝟐) Tension GERİLİMİ PSEUDOPLASTIC Yield Shear thinning PSEUDOPLASTIC PSÖDOPLASTİK Paint Stress PLASTİK PLASTIC Stress NEWTONIAN NEWTONİEN KAYMA Shear Water Shear DILATANT DİLATANT DILATANT Shear thickening Cornstarch KAYMA HIZI Shear Shear RateRate (𝜸, 𝟏/𝒔) ▪ Newtonian fluids are fluids with a constant ratio, 𝜼 (eta) between the shear stress, 𝝉 (tau), and the shear rate, 𝜸 (gamma). 𝜏 𝜂= 𝛾 ▪ Such fluids have the following properties under constant temperature and constant pressure; ▪ The tension formed in the simple shear flow is the shear stress where the two normal tensions differences are zero. ▪ Viscosity does not change with shear rate. ▪ Newtonian fluids ▪ The viscosity is constant while the shearing process is in progress, and the tension in the liquid will immediately go to zero if the shearing is interrupted. ▪ If the measurement is repeated after a time interval, the same value is found again; viscosity does not change over time. ▪ The viscosity values measured at different deformation situations are always simply proportional to each other. ▪ Newtonian flow types: ▪ They obey Newtons Law of Flow. ▪ Their viscosity is independent of shear rate. Shear Stress Viscosity Shear Rate Shear Rate Non-Newtonian Flow ▪ Some fluids deviate from the newtonian flow behavior; therefore, the linear relationship between shear stress and shear rate has been found to be impaired. ▪ Such fluids are referred to as non-newtonian fluids. ▪ For example, in colloidal solutions, emulsions, oinments, clay dispersions, energy may also be needed to break clay mineral flocks or to establish connections between them. ▪ According to this; various flow behaviors and concepts have been introduced depending on the relationship between shear stress and shear rate. a) Dilatant Flow b) Pseudoplastic Flow c) Bingham Plastic Flow a) Dilatant Flow ▪ Such fluids increase in viscosity with increasing shear rate or time. ▪ In the literature, the terms "condensed flow" or "shear thickening flow" are also used. ▪ For example, cornstarch, vinyl resin pastes, sand-water mixtures, 50% solid content suspensions show dilatant behavior. a) Dilatant Flow a) Dilatant Flow a) Dilatant Flow (Slow Motion) b) Pseudoplastic Flow ▪ In pseudoplastic flow, there is a reverse situation of dilatant flow. ▪ Fluids that show a tendency to decrease in viscosity with increasing shear rate are called fluids that show “pseudoplastic” or “thinning flow” behavior. ▪ It is the most common non-newtonian flow type. ▪ Paints, emulsions, and polymeric solutions are examples of such fluids. b) Pseudoplastic Flow a) Dilatant Flow c) Plastic (Bingham) Flow ▪ It is a type of flow that exhibits solid-like behavior under stable conditions. ▪ In order for the flow to start, the shear stress value must be of a certain amount. ▪ This stress value is called yield stress. ▪ After the flow stress value is exceeded, the flow curve of the fluid takes a linear view. c) Plastic (Bingham) Flow c) Plastic (Bingham) Flow ▪ Flow curve does not come the origin value. ▪ No response on low shear rate. ▪ Flocculated suspensions (1-10% solid content), mayonnaise, ointment, toothpaste, ketchup are examples of such fluids, and the yield stress values are greater for these samples. c) Plastic (Bingham) Flow Non-Newtonian Flows  Next Week 10 January, 2025 Summary / Lab Compensations