This is the design of the semi-closed microchip.
The microchip is made of PDMS.
As shown here, the semi-closed microchip has a bath at the middle of the microchannel.
Sealing the bath by oil film, evaporation of the solution can be prevented and the probe type manipulation can be realized with keeping the seal.
To realize the seal by the oil film, the thickness of the oil film is the most important factor for stable seal.