First i think that your protein concentration is to low... A common rule is : protein concentration is good when 50% of your conditions are precipitates and 50% are clears drops. If most of your drops are clear, i think that you must increase your protein concentration (a screen from Hampton can help you for the concentration of your protein: PCT screen). For your oily bubbles, it often appear when PEG is in the condition, it's just phase separation. An other suggestion is that your buffer concentration is too high... If your buffer is a strong buffer, the conditions buffers can be too weak versus your 50mM. By the way, real pH in your condition may be something between the pH of the condition and the pH of the protein sample and not the real pH of your condition. So you don't have a real access to all the pHs of the screens.
In summary, reduce your protein sample buffer concentration (to 25 or 10mM) and increase your protein concentration (to 50, 100 mg/ml or more). Michel.
