Wall-less microfluidics with on-site MHD pumps for continuous flow control

Summary form only given. This paper reports the construction of a novel microfluidic platform without physical walls, i.e. wall-less microfluidics. Different from current microfluidic technology which construct microchannel out of different microfabrication techniques, surface tension is utilized to confine fluid on the hydrophilic patterns which consist of microfluidic network for the wall-less microfluidics. This effect can be schematically demonstrated: the substrate's wettability can be modified with appropriate treatment of the surface, together with photolithography and other fabrication techniques, virtual microchannel can be defined by those hydrophilic patterns that form the microfluidic channels and the hydrophobic parts will be the "walls". In micro scale surface tension plays a key role over other force like inertial force and the surface tension at the interface of the hydrophilic and hydrophobic balances with the inertial force and confines the flow in the hydrophilic zones. For transporting of the liquid on the wall-less microfluidics, magnetohydrodynamic (MHD) pump is used since the structure for which is simple and only pairs of electrodes are needed. By integrating MHD pumps onto the wall-less microfluidics, one can non-invasively control the flow. Two methods have been adopted to pattern the wall-less microfluidics, first one is to make it from PDMS based substrate, virtual channel patterns can be defined by photolithography on a flat PDMS sheet and followed by oxygen plasma treatment, the surface changed to be hydrophilic and liquid can be confined. The second method is to use glass substrate with patterned electrodes for MHD pumps and followed by trichlorosilane treatment to coat a self-assembled monolayer (SAM) layer on the glass substrate so that on chip MHD pumps can be realized. Unlike electrowetting and other methods using Marangoni effect which utilize the surface tension to manipulate droplets, MHD force acts as a body force to continuously move the fluids inside the hydrophilic paths. One of the advantages of wall-less microfluidics is that since there is no momentum consumed on the wall so that the flow resistance is minimized, and less pressure is required for pumping fluid compared with close microchannels. On the other hand, when pumping biological samples, like cells in medium, the effect of the adhesion of biological objects to the physical walls is eliminated. Compared with real wall MHD microfluidics, wall-less MHD fluidics can have higher flow rate and speed up the analysis procedures