In this study, a criterion for normal operation of micro pulsating heat pipes (MPHPs) even in a horizontal orientation is developed through experimental and theoretical investigations. Using MEMS techniques, a ten-turn closed-loop MPHP with overall dimensions of 50 x 33.5 x 1.7 mm(3) is fabricated. The rectangular micro-channels engraved on a silicon wafer have a width of 1 mm and a depth of 0.5 mm. Ethanol is used as the working fluid and the filling ratio is 47% on the average. To allow flow visualization, a glass wafer is bonded to the silicon wafer. The temporal variations in the temperature and pressure of two neighboring vapor plugs in the MPHP are measured using micro-thermocouples and pressure transducers. The flow behavior in the MPHP is recorded using a high-speed camera and fully synchronized in real time with the measured temperature and pressure data. To find a criterion for normal operation of PHPs, the MPHP is treated as a mass-spring-damper system, and it is postulated that the transition from (to) a stopover to (from) a large-amplitude oscillation in the MPHP occurs with negative (positive) damping. This postulate is experimentally confirmed with the MPHP operating in a horizontal orientation. Finally, a figure of merit for normal operation of PHPs is proposed. The ratio of the minimum thermal resistance in a vertical orientation to that in a horizontal orientation (R-th,R- V,R- min/R-th,R- H,R- min) is proportional to (D-h/L-eff)(2), the square of the ratio of the hydraulic diameter to the effective length of PHPs. For PHPs filled with ethanol, it is experimentally confirmed that D-h/L-eff has to be greater than approximately 0.03 for normal operation without any stopovers in a horizontal orientation. (C) 2019 Elsevier Ltd. All rights reserved.