Hemin J Majeed MSc. Pharmaceutical sciences Physical Pharmacy Lab 5- Viscosity Hemin J Majeed MSc. Pharmaceutical sciences

Rheology is a branch of physics that deals with deformation and flow property of matter. Viscosity can be defined as the resistance of a liquid to flow Higher viscous liquid has greater resistance to flow The unit of viscosity is poise (p) and centipoise (cp)

Importance of viscosity in pharmaceutics Drugs absorption from topical product is highly affected by product viscosity The applications of thixotropic liquids in the formulation of pharmaceutical suspensions and emulsions. Fluidity of injected preparation and flexibility of tubing. Flow rate of material during manufacturing process can influence the selection of mixing equipment.

Viscosity measurement The choice of instrument for measuring viscosity of a liquid depends on whether the flow properties of the intended liquid obey Newton’s law of flow

Newton law of flow There is linear relationship between shear rate and shear stress. The rate of shear refers to how fast the liquid flows when a shear stress is applied to it Water, organic solvents and honey can be classified under this type of fluid. Temperature consideration should be taken into account during viscosity determination

Capillary viscometer is used for the measurement of the viscosity of Newtonian system. It is U-shaped glass apparatus with wide arm and a narrow one that contain a capillary tube. There are two marks in the narrow one above and below bulb. This method is applied to find viscosity of unknown in relation to another liquid of known viscosity

Procedure Prepare 50 ml solution of different volume fraction of glycerin in water 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 Measure the viscosity of water and the prepared solutions by capillary viscometer. Fill the viscometer from the wide arm , with the liquid being measured until its level reaches the stop mark. Using a sucker bulb, suck the liquid from the stop mark to the start mark. After that, put your finger on the tip of the viscometer to prevent the liquid from going down , Leave the liquid to descend from start mark to stop mark and record the time. 3. Record three reading for each liquid and take their mean 4. Repeat the above procedure for the unknow liquid.

Calculation The viscosity in (cp) of the liquid under test (ŋ1) can be calculated by measuring the time in (sec) (t1) required to pass between two marks as it flows by gravity through the vertical capillary tube. The liquid of known viscosity (usually water) is (ŋ2)and its time (t2) to pass between two marks can be recorded P1 and p2 are the density of the measured liquid and water respectively

The densities of the prepared solutions can be determined by dividing the mass of 50 ml of liquids by their volumes. Viscosity of water is 0.8904 cp Density of water is 0.997 g/ cm3