Abstract: The invention relates to a device for manipulation of particles (30) in a sample liquid (32) said device comprising a source of ultrasound (16) capable of emitting ultrasound with a given wavelength, an inlet for a sample liquid (2), one or more outlets (4, 5, 6) and a compartment (14), being dimensioned to support a standing ultrasonic wave (40) of said wavelength, characterised in that the device further comprises an inlet for sheath liquid (1, 3) configured to direct a sheath liquid (34) to extend substantially in parallel to an anti-node plane (46) of the ultrasonic standing wave (40) proximate to a sheathed compartment wall. Specifically the device may be used in combination with a particle enumeration device for enumeration of somatic cells in milk.