Pipets containing electrolyte and electrodes

Apparatus comprising:

• a pipet comprising first and second channels separated by a septum and having a tip at which each of the first and second channels is open;
• an electrolyte solution contained within both the first and second channels of the pipet;
• a first electrode extending into the electrolyte in the first channel of the pipet;
• a second electrode extending into the electrolyte in the second channel of the pipet, the second electrode connected to ground;
• a voltage source configured to apply a potential difference between the first and second electrodes;
• a mechanical oscillator configured to apply an oscillatory perturbation to the pipet at a perturbation frequency;
• a current meter configured to measure alternating current components of a current flowing to or from the first electrode;
• and
• a pipet adjustment device responsive to alternating current at the perturbation frequency or at a harmonic of the perturbation frequency detected, by the current meter, to be flowing to or from the first electrode to adjust a position of the pipet so as to control a separation between the tip of the pipet and a surface of interest.