It has been conceived that a charged droplet driven by Coulombic force can be used as a droplet-based microreactor. As a basic research for such applications, electrical charging of a conducting water droplet is studied experimentally. The effects of electric field, medium viscosity, and droplet size are investigated. It is found that the amount of electrical charging increases with the droplet size and the electric field. However, the medium viscosity does not have a significant effect in the range of the present study. A scaling law is derived from the experimental results. Unlike the case of a perfect conductor, the estimated amount of electrical charge (Q(est)) of a water droplet is proportional to the 1.59 power of the droplet radius (R) and the 1.33 power of the electric field strength (E). (For a spherical perfect conductor, Q is proportional to R-2 and E.) In order to understand these differences, numerical simulations are performed for the idealized droplets of perfect conductor. Comparison of the numerical and experimental results suggests that the differences are mainly due to incomplete charging of a water droplet resulted from the combined effect of electrochemical reaction at electrode and the relatively low conductivity of water. (C) 2008 Elsevier Inc. All rights reserved.