A microfluidic device comprises a microfluidic coupon and a fluid reservoir associated with the microfluidic coupon. The fluid reservoir has a vented configuration and a non-vented configuration, and is configured to contain a liquid to be centrifugated. An opening is formed in the fluid reservoir. When the microfluidic coupon is rotated at a target rotational velocity: the opening is open to flow of the liquid when the fluid reservoir is in the vented configuration; and the opening is closed to flow of the liquid when the fluid reservoir is in the non-vented configuration.