Question

Electrophoresis & Sedimentation:

Explain why a teardrop shaped particle would not have a smaller frictional coefficient than a sphere. Why would a teardrop or elipsoid particle not possess superior streamlining compared to a sphere?

Answer 1

In both sedimentation and electrophoresis ,charged particles dispersed in a fluid are separated from it.However, in electrophoresis, an external electric potential is applied to separate the charged species ,but in sedimentation an electric potential is induced.

In both the methods, an electric double layer is created around the particle. According to this double layer theory, all surface charges in on the particles are screened by a diffuse layer of ions, which has the same absolute charge, but of opposite sign (compared to the surface charge).

As the charged particle moves, through a gravitational field(sedimentation) or electric field (in electrophoresis) ,ions in the electric double layer lag behind due to the viscous force of the fluid,thereby creating a net dipole moment (zeta potential in electrophoresis). The sum of all particle dipoles causes sedimentation potential (in sedimentation). The particle moves steadily as the net force acting on it is zero.

While in electrophoresis, the electric field exerts an opposite force on the ions in the diffuse layer compared to that on the surface charge. This former force,due to viscous stress of the fluid retards the particle so is called electrophoretic retardation force.The charged particle is at a steady movement through the diffuse layer,as the total resulting force acting on it is zero .

Due to the electrical double layer of ions,in tear shaped particle,the outer surface ions will face less drag by the viscous stress due to its streamlined shape.So, the difference of electric potential between the diffuse layer and surface will be less which will give low value of zeta potential (in electrophoresis). As the particle moves in the applied electric field owing to this potential ,the speed is slowed.

Hence frictional force , acts in reducing the steady velocity of the particle in tear shaped.So streamlined body has high frictional coefficient( defined by low mobility of particles in electrophoresis.)Sedimentation shows the opposite effects.