Drop on demand printing head and printing method

A drop-on-demand printing head comprising:

• a nozzle assembly comprising:
• a first nozzle connected through a first channel with a first liquid reservoir with a first liquid and having a first drop generating and propelling device for forming on demand a first primary drop of the first liquid and discharging the first primary drop to move along a first path;
• and
• a second nozzle connected through a second channel with a second liquid reservoir with a second liquid and having a second drop generating and propelling device for forming on demand a second primary drop of the second liquid and discharging the second primary drop to move along a second path;
• a reaction chamber;
• wherein the first path crosses with the second path within the reaction chamber at a connection point;
• means for controlling the flight of the first primary drop and the second primary drop and configured to allow the first primary drop to combine with the second primary drop at the connection point into a combined drop so that a chemical reaction is initiated within a controlled environment of the reaction chamber between the first liquid of the first primary drop and the second liquid of the second primary drop;
• and
• at least one gas-supplying nozzle configured to supply gas for controlling flight of the combined drop.