First, I’d like to say that this is not homework, but my research work. I’ll be very appreciated if someone could help me to address this issue, many thanks in advance. My experimental purpose is to generate uniform electric field inside microchannel by externally-applied electric field. (“externally” here means no direct contact between electrodes and water) But there is a questionable point still killing me. As I know, in pure water, autoionization of water molecules can generate hydroxide and hydronium ions at a constant concentration, and these free ions can form Debye space charge layers that screen electric fields. Does it mean, no matter how high DC V is (like 1 KV), the final electric field intensity inside microchannel would go to zero after instantaneous screening process? If the above statement is valid, then I apply alternating current voltage, like sinusoidal AC (100 kHz, peak voltages from 0 and 500 V) instead of DC, I wonder how to calculate electric field intensity inside microchannel? Taken the rate of electrostatic screening into account, what the minimal frequency of ac should I set ? Many thanks again for your attention to this matter.