Measurement of Electric Charge in a Charged Hydroxyapatite Dielectric Using Pendant Drop

A simple
noninvasive measurement method which allows one to determine
the trapped charge in a biocompatible hydroxyapatite dielectric is
developed. The hydroxyapatite samples are charged by electron beam
with energy 30 keV and total irradiated charge ranging from 2 ×
10–9 C to 2 × 10–7 C. The value of the trapped charge is determined
by analyzing the shape change of a liquid droplet hanging from a needle
in proximity of the charged sample surface. The shape change of the
pendant drop in the field of gravity is commonly utilized in the measurements
of the surface free tension (SFT) of liquids. The external electric
field leads to a further modification of the droplet shape and to
an effective change of the SFT. The change of the SFT as a function
of distance between the droplet and sample and the critical distance
at which the droplet detaches from the needle are measured for various
values of the irradiated charge. These two quantities are also derived
theoretically by considering the trapped charge as a single fitting
parameter. We can thus determine the trapped charge in two independent
noninvasive ways. It is noteworthy that our method is easily implementable
into the standard pendant drop setups. As a practical application
of the method, a long-term charge stability of the charged hydroxyapatite
is demonstrated, thus paving the way toward quantitative studies of
its bioactivity in dependence on the value of the trapped charge.