Pressure drops in the flow through micro-orifices and capillaries were measured for silicone oils, aqueous solutions of polyethylene glycol (PEG), and surfactant aqueous solutions The diameter of micro-orifices ranged from 5 mu m to 400 mu m The corresponding length/diameter ratio was from 4 to 0.05 and capillary diameters were 105 mu m and 450 mu m The following results were obtained . silicone oils of 10(-6) m(2)/s and 10(-5) m(2)/s in kinematic viscosity generated a reduction of pressure drop (RPD), that is, drag reduction, similar to the RPD of water and a glycerol/water mixture reported in the previous paper by the present authors When RPD occurred, the pressure drop (PD) of silicone oils of 10(-6) m(2)/s and 10(-5) m(2)/s had nearly the same magnitude. Namely, the difference in viscosity did not influence RPD A 10(3) ppm aqueous solution of PEG20000 provided almost the same PD as that of PEG8000 for the 400 mu m to 15 mu m orifices. but a greater PD than that of PEG8000 for the 10 mu m to 5 mu m orifices A non-ionic surfactant and a cationic surfactant were highly effective in RPD compared with anionic surfactants. the non-ionic and cationic surfactant solutions had PD one order of magnitude lower than that of water under some flow conditions in the concentration range from 1 ppm to 10(4) ppm. but the anionic surfactant solutions did not generate RPD except in the case of the smallest orifice of 5 p.m in diameter The PD of the non-ionic surfactant solution showed a steep rise at a Reynolds number (Re-t) for 400 mu m to 15 mu m orifices The Re-t provides the relationship Re-t = K/D. where D is the orifice diameter. and K is a constant of 2 x 10(-2) m for the 100-20 mu m orifices irrespective of liquid concentration. Capillary flow experiment revealed that the PEG, non-ionic and cationic surfactant solutions generated RPD also in a laminar flow through the capillary of 105 mu m in diameter, but not in the flow through the capillary of 450 mu m in diameter In order to clarify the cause of RPD, an additional experiment was carried out by changing the orifice material from metal to acrylic resin The result gave a different appearance of RPD. suggesting that RPD is related to an interfacial phenomenon between the liquid and wall The large RPDs found in the present experiment are very interesting from both academic and practical viewpoints. (c) 2010 Elsevier B V. All rights reserved