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February 2015 • SMT Magazine 39 of such needles remains obscure, although the shape of their pedestals is explained as the so- called Taylor cones representing an equilibrium (between the electrostatic polarization and the surface tension) geometry of a metal surface. From the perspectives of this paper's philoso- phy, these needles, metal whiskers, and those responsible for shunting and solid state mem- ory features are all mutually related due to the electrostatic energy gain. 8. Because of its counterintuitive nature, the discussion about strong electric fields in the near surface regions of real metals may require further comment. These fields can arise from spatial variations of the work function depend- ing on multiple imperfections in structure and composition. At the first glance, the existence of such fields may appear contradictory. Indeed, free electrons in metals have a tendency to screen electric charge fluctuations, thereby sup- pressing electric fields. However, as explained in the next section, the role of free electrons is exact opposite. Their ability to move underlies the electric charge variations. To avoid any misunderstanding, it is not the condition of local electroneutrality, but rather that of minimum free energy that determines the electric charge (and other parameters) dis- tributions. The electrons will always move to minimize the system free energy; this leads to non-uniform charge distributions in non-ho- mogeneous systems. In other words, not only the existence of surface electric fields and their corresponding electric charge variations are ful- ly consistent with the concept of free electrons, but it is due to free electrons that different local regions of a metal can exchange electric charges minimizing the system free energy. It is widely known in the physics of semiconductors (but has not been applied to metals often) that it is not the electric potential j, but rather the elec- Figure 11: Structures pulled up by a strong electric field on the surface of a molten metal [3] . Figure 10: illustrations of the electric field induced conductive filaments in resistive memory devices. eLeCTrOSTaTIC MeCHaNISM OF NuCLeaTION aNd GrOWTH OF MeTaL WHISKerS continues Feature