Basic Single Particle Measurements

Direct measurements of forces acting on a microparticle are made by using the dc field of an electrostatic balance or an electrodynamic balance to counteract any vertical applied force, the most obvious force being gravity. But numerous other forces are of interest, including the aerodynamic drag force, F_d, the radiometric force, F_r, phoretic forces, F_ph and magnetic forces, F_H. Since the ac field exerts no time-average force on the levitated particle when it is at the null point of the balance chamber, a vertical force balance on a suspended particle yields 4.1$$ {C_0}q{V_{dc}}/{z_0} = - mg + {F_d} + {F_r} + {F_{ph}} + {F_H}, $$ where C₀ and z₀ are geometrical parameters (see Sect. 2.5), q is the Coulombic charge, V_dc is the levitation voltage, and mg is the particle weight. In the absence of forces other than gravity any particular force can be measured in terms of the particle weight by two simple experiments. If V₀ is the dc voltage required to suspend the particle when only the gravitational force acts on it, Eq. (4.1) reduces to 4.2$$ {C_0}q{V_{dc}}/{z_0} = - mg, $$