In Fig. 11, *E*^{2} at the center line (*x* = 0.00 mm) is highest in the vicinity of the ground electrode and decreases along the *y*-axis through the slot in the high-voltage electrode. There are localized regions of high electric field strength near the slot edges due to the corners, and these are indicated by the sudden increase in *E*^{2} at location *x* = 0.22 mm. At *x* = 0.50 mm, the value of *E*^{2} increases along the *y*-axis until *y* = 1.0 mm. Here, there is a discontinuity in the curve since the high-voltage electrode surface is reached. With $E2$ calculated, the next step in the analysis was to determine its gradient by differentiating the curves in Fig. 11 with respect to the *y*-coordinate. However, with the above-mentioned values of permittivity and electric field gradient known, the only remaining unknown in Eq. (2) was the bubble radius, *a*. For nucleate boiling, the bubble radius is generally a function of liquid and vapor density, surface tension, wall superheat, and heat flux. As a result, bubble departure diameter varies during the pool boiling (or liquid film flow boiling) regime. However, since the purpose of this study was simply to determine the order-of-magnitude of the DEP force acting on the vapor bubbles, a fixed bubble departure diameter was chosen. As a first approximation, the expected bubble departure diameter, *D*_{d}, for given heat flux and superheat was estimated by the well-known correlation by Zuber [22] for nucleate boiling
Display Formula

(7)$Dd=[6\sigma kl\Delta Tg(\rho l\u2212\rho v)q"]13$