Colloid electrospray thrusters are unique amongst electric thrusters due to their inherent scalability and the possibility of covering a broad range of Isp and thrust. The main research fronts are finding a suitable propellant, improving their mi- crofabrication, and understanding the underlying physics. In this work, a series of experiments have been carried out with a single emitter capillary electrospray with the ionic liquid EMI-BF4 and how the operational variables affect its per- formance has been studied. A breakdown of all the loss mechanisms that make up for the total efficiency of a colloid thruster is also presented. The past work on colloid thrusters was reviewed, along with the basic physics of electrosprays and the experimental techniques used to characterize them. It is observed that for the broad ranges of flow rates covered in the experiments, EMI-BF4 with a capillary emitter always emits a large proportion of ions along with some droplets. At minimum flow rate, the droplets are still present, causing the efficiency to be low at all the flow rates studied. The effect of varying the emit- ter temperature and voltage is studied, concluding that raising their value would increase slightly the current but have little impact on the efficiency and Isp. Des- pite it became clear that EMI-BF4 is not suitable for its use in colloid thrusters, it lead to identifying what are the desired characteristics of a favorable propellant for these kinds of thrusters. These include having electrochemical reaction products that can be drained from the emitter and a very high ion evaporation energy to prevent ion emission. The efficiency of a colloid thruster was analyzed and each loss mechanism was iden- tified. A method to calculate the losses from electrochemical reactions is presented. Identifying each factor of the overall efficiency assists the optimization of the ef- ficiency and the assessment of the suitability of a thruster by studying the main loss mechanisms
