Historically, microfluidics was a mere size reduction of well-established laminar macrofluidics. Microflows were confined inside solid wall channels and moved by pumps or syringes. Progressively new ways of doing microfluidics have emerged. Digital and droplet microfluidics are two techniques that have the advantage of confining targets such as biological objects of interest inside droplets of picoliter to nanoliter volumes. Recently, with the improving knowledge of capillary forces, it has been found that it is not always a necessity that microflows be confined between solid walls. Hydrophilic rails  or posts  could be sufficient to guide a micro flow. The main advantage is to have direct access to the flowing liquid. The second advantage is the large liquid-gas surface area. This type of microfluidics is usually called “open microfluidics” or sometimes “virtual walls microfluidics”, or HCMs for “hydrodynamically confined micro-flows.” Open microfluidic systems are now common in nanotechnology, biotechnology, fuel cells, and space technology [3–5].
From a general point of view, open microfluidics is a general designation for liquids having one interface with air (fig. 8.1). There is a wide diversity in this family. It encompasses droplets pierced by wires , liquid bridges, and liquid spreading between rails and inside grooves . Still more recently, new concepts have emerged, such as paper-based microfluidics – where liquid ...