Growth of the KHI at the boundary of the jet and coronal plumes. (a): jet seen in EUI 174 Å passband before the development of ruffles, while the trajectory of the jet is smooth (the dashed line in blue). The location of coronal plumes is outlined by the dashed line in red (see Appendix B for further information). The acute angle between the upstream of the jet and the plumes is about 38.5°. (b): jet seen in EUI 174 Å passband while the ruffles have developed and the boundary becomes sawtooth (see the dashed line in blue), indicating the occurrence of the KHI. The red arrow points to a ruffle, whose distortions are tracked and given in panel d. (c): edge-enhanced image of that shown in panel b, based on Sobel operator, on which the edges of the jet and the developed ruffles are better seen. (d): growth of the distortions of the ruffle marked in panel b. The ruffle is tracked in five frames of the observations and its distortions (the asteroid symbols with error bars) follow an exponential growth that is 326.2e^0.061t, where t is the time in seconds.