16 and aqueous glycerol 17 are denoted as w 2 and w 1 respectively the holdup of the nacl 16 e 2 is the ratio of the area occupied by the nacl 16 to the whole area of the cross section of the channel 20 as the height is common this becomes similarly the holdup of the aqueous glycerol 17 is e 1 1 u2212 e 2 when the two fluids 16 17 are in contact with the wall of the channel 20 the negatively charged surface 20 will influence the distribution of free ions in the nacl solution 16 to form an electrical double layer near the channel wall 20 but the aqueous glycerol 17 will only minimally form an electrical double layer as there are few free ions thus the electroosmotic flow will only affect the nacl solution 16 when a positive voltage is applied between a and b a as the positive electrode b as the negative electrode the electroosmotic force will force the nacl solution 16 to flow in the same direction as the pressure driven flow if the negative electric field is applied a is negative b is positive an opposite electroosmotic flow will result which is against the pressure driven flow a fluorescent dye such as for example fluorescein disodium salt c 20 h 10 na 2 o 5 also called acid yellow 73 may be added to the nacl solution 16 for image collection when the fluorescein is illuminated by a mercury lamp a coupled charge device ccd camera or other similar device may be used for image capturing to enable measurements to be taken the same volumetric flow rates of the two inlet flows a and c may be ensured through the use of identical.