SURFACE TENSION

SURFACE TENSION. 

                         Surface tension is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension. The magnitude of this force per unit length of the free surface will have the same value as the surface energy per unit area. It is denoteby Greek letter 𝛔 (called sigma) . In MKS units, it is expressed as kgf/m while in SI units as N/M.

SURFACE TENSION

               The phenomenon of surface tension is explained by above Figure. Consider three molecules of A, B, C of a liquid in a mass of liquid. The molecule A is attracted in all directions equally by the surrounding molecules of the liquid. Thus the resultant force acting on the molecule A is zero . But the molecule B, which is situated near the free surface, is acted upon by upward and downward forces which are unbalanced. Thus a net resultant force on molecule B is acting in downward direction. The molecule C situated on the free surface of the liquid does experience a resultant downward force. Thus the free surface of the liquid acts like a very thin film under tension of the surface of the liquid acts as though it is an elastic membrane under tension. 

SURFACE TENSION ON LIQID DROPLET.

                               Consider a small spherical droplet of a liquid of radius ‘r’ . On the entire surface of droplet, the tensile due to surface tension will be acting. 
   Let 𝛔 = Surface tension of the liquid
           p = Pressure intensity inside the droplet (in excess of the outside pressure intensity)
           d = Dia. of the droplet.

SURFACE TENSION ON LIQUID DROPLET

                                        The End. 

    Please comment if you have any query. 

Disclaimer : An attempt has been made to make this blog error free. If there is any error, please forgive.

       



                                                           Thank you. 

Newton's Law of Viscosity

NEWTON'S LAW OF VISCOSITY. 

             It states that the shear stress(τ ) on a fluid element layer is directly proportional to the rate of shear strain. The constant of proportionality is called the co-efficient of viscosity. Mathematically, it is expressed as given by equation 

       
                               τ  =  μ (du/dy)

                Fluid which obey the above relation are known as Newtonian fluids and the fluids which do not obey the above relation are called Non-Newtonian fluids.

 VARIATION OF VISCOSITY WITH TEMPERATURE. 

                                Temperature affects the viscosity. The viscosity of liquids decreases with the increase of temperature while the viscosity of gases increases with the increase of temperature. This is due to reason that the viscous forces in a fluid are due to cohesive forces and molecular momentum transfer. In liquids, the cohesive forces predominates the molecular momentum transfer, due to closely packed molecules and with the increase in temperature, the cohesive decreases with the result of decreasing viscosity. But in case of gases the cohesive forces are small and molecular momentum transfer predominates. With the increase in temperature, molecular momentum transfer increases and hence viscosity increases.

TYPES OF FLUIDS. 

                              The fluid may be classified into the following five types :

1. Ideal fluid, 

2. Real fluid, 

3. Newtonian fluid

4. Non-Newtonian fluid, and 

5. Ideal plastic fluid. 

1.  IDEAL FLUID.  

                       A fluid, which is incompressible and is having no viscosity, is known as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluids, which exists, have some viscosity. 

2.  REAL FLUID. 

                    A fluid, which possesses, is known as real fluid. All the fluids, in actual practice, are real fluids.

3. NEWTONIAN FLUID. 

                   A real fluid, in which the shear stress is directly proportional to the rate of shear strain (or velocity gradient) , is known as a Newtonian fluid. 

TYPES OF FLUID

4.  NON-NEWTONIAN FLUID. 

                  A real fluid, in which the shear stress is not proportional to the shear strain (or velocity gradient) , known as a Non-Newtonian fluid. 

5.  IDEAL PLASTIC FLUID. 

                   A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain ( or velocity gradient) , is known a ideal plastic fluid. 

VISCOSITY

VISCOSITY

        Viscosity is defined as the property of a fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid. When two layers of a fluid, a distance ‘dy’ apart, move one over  the other at different velocities, say  ‘u’ and  ‘u + du’ as shown in below figure, the velocity together with relative velocity causes a shear stress acting between the fluid layers. 

         The top layer causes a shear stress on the adjacent lower layer while the lower layer causes a shear stress on the adjacent top layer. This shear stress is proportional to  ‘y’ . It is denoted by symbol ‘tau’.

   

  Units of Viscosity 

          

    MKS unit of viscosity = kgf -sec/m²

     CGS unit of  viscosity = dyne -sec/cm²

      SI unit of viscosity = Newton - sec/ m²

    

                  The unit of viscosity in CGS is also called Poise which is equal to  dyne -sec/cm².

                Note - 

                          1. If viscosity is given in poise, it must be divided by 10 to get its equivalent numerical value in SI units. 

            The viscosity of water at 20°C is 0.01 poise or 1.0 centipoise. 

KINEMATIC VISCOSITY 

                          It is defined as the ratio between the dynamic viscosity and density of fluid. It is denoted by the Greek symbol called ‘nu’.

UNITS OF KINEMATIC VISCOSITY

                                 In MKS and SI, the unit of kinematic viscosity is ‘metre²/sec’ while in CGS units it is written as ‘cm²/sec’. In CGS units, kinematic viscosity is also known as ‘stoke’.

                                            The End. 

    Please comment if you have any query. 

Disclaimer : An attempt has been made to make this blog error free. If there is any error, please forgive.

       



                                                           Thank you. 

   

PROPERTIES OF FLUIDS

INTRODUCTION

                        Fluid mechanics is that branch of science which deals with the behavior of the fluids (liquids or gases) at rest as well as in motion. 

PROPERTIES OF FLUIDS

    DENSITY OR MASS DENSITY : 

                 It is defined as the ratio of the mass of a fluid to its volume. 

                OR

   Mass per unit volume is called density. 

                 It is denoted by symbol 'rho'. 

   The value of density of water is 1000kg/m³ or       1gm/cm³.

SPECIFIC WEIGHT OR WEIGHT DENSITY

                It is the ratio between the weight of a fluid to its volume. 

  The value of specific weight or weight density for water is 9.81 × 1000 Newton/m³.

SPECIFIC VOLUME

            It is defined as the volume of a fluid occupied by a unit mass or volume per unit mass is called specific volume. 

    Thus specific volume is the reciprocal of mass density. It's is expressed as m³/kg. It is commonly applied to gases. 

  

SPECIFIC GRAVITY

           It is defined as the ratio of the weight density (or density)of a fluid to the weight density (or density) of a standard fluid. For liquids, the standard fluid is taken water at 4°C and for gases, the standard fluid is taken air. 

 Specific gravity is also called relative density. 

 If the specific gravity of a fluid is known, then the density of the fluid will be equal to specific gravity of fluid multiplied by the density of water. 

For example, the specific gravity of mercury is 13.6, hence density of mercury = 13.6×1000 = 13600 kg/m³.

                                   The End

    Please comment if you have any query. 

Disclaimer : An attempt has been made to make this blog error free. If there is any error, please forgive.

         



                                                           Thank you.