We now consider the case of grain charging by plasma collection currents when the grain density in the plasma is high. In this case, the grain charge may be lower than its "isolated" value, as discussed earlier. There are two competing effects that lead to this result; one is that the capacitance of the grain increases, which tends to increase the charge, and the other is that the magnitude of the grain surface potential relative to the plasma potential decreases, which decreases the charge. The capacitance of a grain increases from its value in vacuum (where the capacitance is proportional to the grain radius) as the grain spacing becomes comparable to or less than the Debye length. In this case, the positive sheath (for a negatively charged grain) moves closer to the grain surface, thus essentially decreasing the capacitor gap, or the distance between the edge of the sheath and the grain surface, thereby increasing the capacitance (Whipple et al 1985, Northrop 1992) - Mendis, D. A.; Rosenberg, M., "Cosmic Dusty Plasmas
" (1994) Annual Review of Astronomy and Astrophysics
, Volume 32, 1994, see "2.2 Grain Ensemble", page 426.