Process to control fluid flow in a microfluidic device (20, 80, 160), the microfluidic device (20, 80, 160) comprising: a substrate; a microfluidic channel structure (30, 162, 240, 260, 400, 500) formed on the substrate, the channel structure (30, 162, 240, 260, 400, 500) including a reservoir (164, 214) and a first channel (242, 262, 402, 502) extending from the reservoir (164, 214); and first (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B) and second (34, 244A, 244B, 264C, 404C, 404D, 504C) fluid actuators positioned within the first channel (242, 262, 402, 502), whereby the process comprises: - operating the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B) in a first position to selectively cause general fluid flow in a first direction (37, A) from the reservoir (164, 214) into the first channel (242, 262, 402, 502); - operating the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C) in a second position to selectively cause reverse fluid flow in a second direction (38, B) and is characterized by: - maintaining, during activation of the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C), the general fluid flow in the first direction (37, A) due to continued activation of the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B). a substrate; a microfluidic channel structure (30, 162, 240, 260, 400, 500) formed on the substrate, the channel structure (30, 162, 240, 260, 400, 500) including a reservoir (164, 214) and a first channel (242, 262, 402, 502) extending from the reservoir (164, 214); and first (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B) and second (34, 244A, 244B, 264C, 404C, 404D, 504C) fluid actuators positioned within the first channel (242, 262, 402, 502), whereby the process comprises: - operating the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B) in a first position to selectively cause general fluid flow in a first direction (37, A) from the reservoir (164, 214) into the first channel (242, 262, 402, 502); - operating the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C) in a second position to selectively cause reverse fluid flow in a second direction (38, B) and is characterized by: - maintaining, during activation of the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C), the general fluid flow in the first direction (37, A) due to continued activation of the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B). - operating the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B) in a first position to selectively cause general fluid flow in a first direction (37, A) from the reservoir (164, 214) into the first channel (242, 262, 402, 502); - operating the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C) in a second position to selectively cause reverse fluid flow in a second direction (38, B) and is characterized by: - maintaining, during activation of the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C), the general fluid flow in the first direction (37, A) due to continued activation of the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B). - maintaining, during activation of the second fluid actuator (34, 244A, 244B, 264C, 404C, 404D, 504C), the general fluid flow in the first direction (37, A) due to continued activation of the first fluid actuator (32, 244C, 264A, 264B, 404A, 404B, 504A, 504B).