Experimental study of mass transfer in water/ionic liquid microdroplet systems using micro-LIF technique

• Incorporated ionic liquid into microdroplet system.
• Adapted micro-LIF for visualization of liquid–liquid droplet-based processes.
• Revealed the interface mass transfer process in water/ionic liquid microdroplet systems.
• Determined mass transfer coefficients at droplet formation and droplet moving stages.
• Disclosed enhanced mass transfer of ionic liquid in droplet microsystem.

Experimental study of mass transfer in water/ionic liquid (IL) microdroplet systems was carried out using micro-LIF (laser-induced fluorescence) measurement method, where [C4mim][PF6] was adopted as a representative IL and Rhodamine 6G as the model solute. Firstly, microfluidic devices and auxiliary reference experiments were elaborately designed to facilitate concentration measurement by micro-LIF in liquid–liquid droplet flow systems. The dynamic mass transfer behaviors of Rhodamine 6G from water droplet to surrounding IL in two microchannels were then observed vividly, and quantitatively characterized by mass transfer coefficients kLakLa. The results showed the droplet-based microsystems brought evident enhancement on the mass transfer between IL and water, where kLakLa was in the range of 0.05–1.5 s−1, about 103–104 times higher than that in a conventional macrosystem. This work disclosed the mass transfer behavior in water/IL microdroplet system and confirmed the great potential of droplet-based microsystem in intensifying the mass transfer in IL-involved multiphase systems.