Syringe pumps,which provide precise and steady flow rates,are widely used in microfluidic research and applications.However,most syringe pumps are mechanically driven and the advancing of the mechanical parts inevitably introduce fluctuations or pulses to the fluid flow,thus affecting the steadiness of the flow.However,the resulting fluctuation in flow rates is normally too low to notice.To the best of our knowledge,there is no experimental evidence confirming the effect of syringe pumps.Here we introduce a robust visual detection of the unsteadiness induced by the stepping motor in a syringe pump,in the form of ripples on the interface of an aqueous two-phase system with an ultralow interfacial tension.We use a typical glass capillary device in our experiments.A cylindrical glass microcapillary,with outer diameter of 1.0 mm,is tapered to the desired diameter,typically around 50μm,and then co-axially aligned with a square capillary,whose inner dimension is 1.05mm.We observe the ripples with an inverted microscope (江南XD-101) equipped with a high speed camera (MotionPro X4) at typically 4000 frames per second.We observe the vibration at a fixed location,and obtain the frequency spectrum of the vibration by applying image analyses using a Discrete Fourier Transform.The ripples are found to exhibit the same frequency as that delivered by the stepping motor of the syringe pump which drives the inner fluid,named as fpump,for various flow rates Q,syringe diameters D and advancing step sizes s,as shown in equation (1).fpump = 4Q/(πD2s) (1) The experimental results suggest that the low interfacial tension system can reflect the disturbance induced by the inner pump,providing an insight into understanding the fluctuation that syringe pumps induces.Our work can potential offer a new way to quantify and compare the unsteadiness of syringe pumps.