this film then being able to form an electrical resistance to give orders of magnitude depending on the size of the droplet the liquid used and the experimental conditions the time it takes for a droplet to evaporate is from a few seconds to a few tens of seconds the volume of the droplet being between 1 u03bcl and 20 u03bcl or even more various liquids like tris tris hydroxymethyl aminomethane or purified water may be used tris has the advantage of being a saline solution that allows bacteria to be preserved for a few days it is therefore widely used as a biological buffer it is preferable for the slide carrying the droplet not to be too hydrophobic or even hydrophilic as will be showed later on the process works with various types of particle mention is made by way of example of 1 u03bcm diameter silanol microspheres 1 u03bcm diameter latex microspheres and bacteria conclusive tests were carried out on e coli or bacillus subtilus bacteria the process works with a wide range of concentrations from one particle per droplet up to one hundred thousand particles per droplet it is of course preferable for the illumination coming from the light source to be as homogenous as possible in other words the surface of the meniscus is illuminated with an intensity that is substantially equal at each point it is also preferable for the illumination to have a certain spatial coherence that is to.