Viscosity of Liquids Essay
The goal of this experiment was to determine the viscosity of given liquids. Two different methods were employed, the first measures time of flow of several methanol-water solutions, from point A to point B. The second method involves dropping a foreign object, in this case a sphere, into a cylinder of glycerol and measuring the time it takes for it to travel a specific distance down the tube. The viscosity of a 0%, 20%, 40%, 60%, 80% and 100% methanol by volume solutions was measured to be 0.89, 1.28, 1.
53, 1.46, 1.11 and 0.54±0.001P, respectively.
The falling sphere method was performed under two different temperatures. At 5.7°C the viscosity of glycerol was calculated to be 29.8±0.
1P and at 22.7°C it was 6.3±0.1P.Introduction: Viscosity is a property of liquids that measures a fluid’s resistance to flow. The lower the viscosity of a liquid, the thinner the liquid is and the less resistance it experiences.
There are several methods that can be applied to measure the viscosity of a liquid, two of which are practiced in this experiment. The first part of the experiment uses an Ostwald viscometer to determine how long it takes a liquid to flow through the capillary tube of the viscometer. Poiseuille’s law demonstrates that the laminar flow of a liquid through a small tube is proportional to the fourth power of the tube’s radius, which is employed in the first method. Poiseuille’s law is given in equation 1, in which dV/dt is the volume flow rate, r is the radius, L is length of the tube, ?P is the pressure difference across the ends of the tube and ? is the viscosity.After integrating and rearranging equation 1, a simpler equation is obtained for viscosity which is given by equation 2, in which t is the length of time for a liquid of volume V, to flow through a capillary tube of length L and radius r.
The parameters r, V, and L remain constant for a given viscometer and thus all of the constants can be combined to give a cell constant, B, for the viscometer. Since pressure is proportional to the density ?, of the liquid, equation 2 can be simplified even further to give equation 3.In order to properly use equation 3, the viscometer must be calibrated and B must be found. This is done so by measuring the time of flow of a liquid of known viscosity and density, in this case pure water. This method for determining viscosity of a liquid is more useful for less viscous substances.For fluids that are thicker; more viscous, a different method is used more effectively.
This alternative method involves measuring the rate of drop of spheres in a specific fluid. When the sphere is dropped, it has an initial acceleration due to gravity. However at some point, an equilibrium state is reached when the buoyant force and drag force balance out the gravitational force. At this point, the sum of the forces is zero and the ball is traveling at a constant velocity called the terminal velocity.
Stoke’s law represents this behavior of a small sphere falling through a cylindrical tube filled with a viscous liquid. His law is shown in equation 4 in which g is the acceleration due to gravity, t is the time required for the sphere of radius r to drop a distance L, and ?s and ?L are densities of the sphere and fluid respectively.Equation 4 is modified to account for “end” and “wall” effects which are caused by the finite dimensions of the cylinder. This modified form of equation 4 is given in equation 5, in which x is the ratio of the diameter of the sphere to that of the cylinder, and y is the ratio of the sphere diameter to the total height of the liquid in the cylinder.Experimental: In part one of this experiment, a size #100 Ostwald viscometer was cleaned and dried by rinsing it with water and then acetone. Suction from the aspirator was used to remove any remaining acetone.
A pipette was used to add 5mL of water to the wide side of the viscometer, which was then clamped inside a 25°C water bath and allowed to equilibrate for 5-10 minutes, keeping both fiducial marks below the water bath level.A pipette bulb was used to pull the water up the capillary tube until it reached the second reservoir. A stop watch was started when the water meniscus reached the top mark, and stopped when it reached the bottom mark. The same calibration was repeated twice more to obtain an error for time. Next, solutions of methanol-water were made that were 20%, 40%, 60%, 80% and 100% methanol by volume. The methanol was obtained from Pharmaco-AAPER.
The same rate of flow procedure performed on the water was performed on the methanol solutions.For part two of this experiment, the viscosity of glycerol was measured. A 250mL graduated cylinder was filled with glycerol, obtained from Fischer Scientific, until the level of glycerol was 3cm up the tubing in the stopper that was in place on top of the cylinder. The temperature as well as the height of the glycerol was measured. The mass of 3/32 inch stainless steel and Teflon spheres was measured by weighing out approximately 10 spheres and dividing by the number of spheres to obtain the mass of a single sphere.
In order to measure the velocity of the spheres as they fell through the liquid, two marks were made on the cylinder and the distance between them was measured in cm.One of the Teflon spheres was dropped into the tube on the top of the cylinder and a long tipped pipette was used to push the sphere until it started to descend on its own due to gravity. The time it took for the sphere to fall from the top to bottom marks of the cylinder was measured, and this procedure was repeated with two more Teflon spheres. The same measurements were performed on three stainless steel spheres. A water bath was then filled with ice water and the cylinder was placed inside for 20-30 minutes allowing it to equilibrate.
The temperature of the glycerol was recorded and the same procedure with the spheres was performed on 1/16 inch stainless steel and brass spheres.