A microfluidic device comprising a first surface portion and a first sample retaining element, which have differing affinities to a fluid, and a method comprising supplying a sample to such a device. In some embodiments, the differing affinity is a result of plasma, ion embedding, surface charging, chemical, optical, electronic and/or electromagnetic treatment. Also, a microfluidic device comprising at least one microcapillary device having a sample retaining element, at least one surface of which exhibits hydrophobicity, hydrophilicity, electromagnetic force exertion and electrostatic force exertion. Also, a microfluidic device comprising a first element having a hydrophilic pattern comprising at least a first sample retaining element. Also, a method comprising supplying a sample to a channel between a first element and a second element, and inducing in the first element at least one hydrophilic pattern by electrets or by internal or external electrodes to provide a charged surface.