A flow cell for the ultrasonic treatment of a liquid includes a housing with a wall at least partially defining a flow chamber therein. The housing includes a first liquid passage and a second liquid passage for flowing the liquid into and out of the flow chamber. An ultrasonic horn extends into the flow chamber for imparting ultrasonic energy to liquid in the flow chamber, the ultrasonic horn having an outer surface and an end surface, and having a recess formed in the end surface. A tubular member having a first end in fluid communication with a second end, is arranged such that the first end is in fluid communication with the second liquid passage, and the second end extends into the recess in the ultrasonic horn.