2ViscosityViscosity is an internal property of a fluid that offers resistance to flowIf the fluid has a high viscosity it strongly resists flow,If the fluid has a low viscosity, it offers less resistance to flow
3How to measure viscosity There are many ways to measure viscosityThe falling-sphere method is a simple way to study viscosity fluids at varying temperatures
4Falling-sphere method A cylinder is filled with fluid and two marks placed at the ends of the distanceYou drop a sphere into a liquid and measure the time required for the sphere to travel a calibrated distanceLayers of fluid molecules sliding over other layers with type of frictional forces depending on the viscosity. Fluid stationary at wall moves with increasing velocity away from wall
5THEORYThe force needed to separate molecules of the fluid according to Stokes is :F = 6(pi)RnvcR: radius of the spheren: viscosity of the fluidvc: velocity of the sphereThis force can be set equal to the gravitational force modified to account for the buoyant effect as follows
6pS : the density of the sphere THEORY6 (pi) R n vc = 4/3 (pi) R3 (pS-pL) gpS : the density of the spherepL : the density of the liquidg : acceleration due to gravity
7THEORYvelocity can be set equal to L/t (distance/time) and the equation solved for n asn = [2 g R2 (pS-pL) t] / 9L
8THEORYThe velocity must be modified for the compression of the fluid by the cylinder wallsvc = v ( x)x is the ratio of sphere diameter to cylinder diameter
9THEORYThe velocity must also be modified for the finite falling distance byvc = v ( y )y is the ratio of sphere diameter to total liquid height
10THEORYvelocity can now be described asvc = v ( x) ( y).
11n = [2 g R2 (pS-pL) t] / [9L (1 + 2.4x) (1 + 1.65y)]. THEORYThe new values for vc can now be substituted into equationn = [2 g R2 (pS-pL) t] / [9L ( x) ( y)].Different size particles yield dramatically different settling velocities.all variables necessary to calculate n can be measured directly except for density, which can be easily calculated from measurements .
12Estimated Cost 2m Glass Tube: $10 Plastic Sphere: $0.5 (we might use more than one sphere with different densities depends on the estimated viscosity of the fluid)Stop Watch: $0