A method that includes introducing a first fluid at a first flow rate into a first end of a channel of a hydraulic energy transfer system, introducing a second fluid at a second flow rate into a second end of the channel opposite the first end, wherein the first flow rate is lower than the second flow rate, and operating the hydraulic energy transfer system to output a third fluid comprising the first fluid and a portion of the second fluid and exhibiting a step-change in concentration of the first fluid. The first fluid is proppant slurry introduced at a first pressure, the second fluid is a clean fluid introduced at a second pressure higher than the first pressure, and third fluid is a fracturing fluid exhibiting the step-change in proppant concentration. The hydraulic energy transfer system includes a rotary isobaric pressure exchanger.