Skip to main content

capillary rise

A liquid of density ρ and surface tension σ rises in a capillary of inner radius r to a heighth=2σcos⁡θρgrwhere θ is the contact angle made by the liquid meniscus with the capillary’s surface.

Capillary Rise




The liquid rises due to the forces of adhesion, cohesion, and surface tension. If adhesive force (liquid-capillary) is more than the cohesive force (liquid-liquid) then liquid rises as in case of water rise in a glass capillary. In this case, the contact angle is less than 90 deg and the meniscus is concave. If adhesive force is less than the cohesive force then liquid depresses as in case of mercury in a glass capillary. In this case, the contact angle is greater than 90 deg and the meniscus is convex.

The formula for capillary rise can be derived by balancing forces on the liquid column. The weight of the liquid (πr2hρg) is balanced by the upward force due to surface tension (2πrσcos⁡θ). This formula can also be derived using pressure balance.

The capillary rise experiment is used to measure the surface tension of a liquid.

 

Labels:

Comments

Post a Comment

Popular posts from this blog

Fluid Kinematics

  Fluid Kinematics Fluid Kinematics  deals with the motion of fluids such as displacement, velocity, acceleration, and other aspects. This topic is useful in terms of the exam and the knowledge of the candidate. Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces the cause the motion. Types of Fluid Flows Fluid flow may be classified under the following headings; Steady & Unsteady Flow Uniform & Non-uniform Flow Steady uniform flow Conditions do not change with position in the stream or with time. E.g. flow of water in a pipe of constant diameter at a constant velocity. Steady non-uniform flow Conditions change from point to point in the stream but do not change with time. E.g. Flow in a tapering pipe with the constant velocity at the inlet. Unsteady uniform flow At a given instant in time the conditions at every point are the same but will change with time. E.g. A pipe of constant diameter connecte
Post a Comment
Read more

THE NO-SLIP CONDITION

 THE NO-SLIP CONDITION Fluid flow is often confined by solid surfaces, and it is important to understand how the presence of solid surfaces affects fluid flow. We know that water in a river cannot flow through large rocks, and must go around them. That is, the water velocity normal to the rock surface must be zero, and water approaching the surface normally comes to a complete stop at the surface. What is not as obvious is that water approaching the rock at any angle also comes to a complete stop at the rock surface, and thus the tangential velocity of water at the surface is also zero. Consider the flow of a fluid in a stationary pipe or over a solid surface that is nonporous (i.e., impermeable to the fluid). All experimental observations indicate that a fluid in motion comes to a complete stop at the surfaceand assumes a zero velocity relative to the surface. That is, a fluid in direct contact with a solid “sticks” to the surface, and there is no slip. This is known as the no-slip
Post a Comment
Read more

Internal versus External Flow

Internal versus External Flow A fluid flow is classified as being internal or external, depending on whether the fluid flows in a confined space or over a surface. The flow of an unbounded fluid over a surface such as a plate, a wire, or a pipe is external flow. The flow in a pipe or duct is internal flow if the fluid is completely bounded by solid surfaces. Water flow in a pipe, for example, is internal flow, and airflow over a ball or over an exposed pipe during a windy day is external flow (Fig. 1–18). The flow of liquids in a duct is called open-channel flow if the duct is only partially filled with the liquid and there is a free surface. The flows of water in rivers and irrigation ditches are examples of such flows. Internal flows are dominated by the influence of viscosity throughout the flow field. In external flows the viscous effects are limited to boundary layers near solid surfaces and to wake regions downstream of bodies. 
Post a Comment
Read more