The phrase "fluid line between" is correct and usable in written English.
You can use it to describe a situation or relationship that is ambiguous and changes over a period of time. For example, "There is a fluid line between right and wrong in this context."
Mr. Huyghe, from Paris, makes a striking New York solo debut with two recent short films that explore the fluid line between life and fiction.
The fluid line between our inner and outer worlds -- between the objective and the subjective -- is one of the critical cultural issues of our time.
The film's exquisite tag ending, together with the title that follows it — "For Kris," who is Swanberg's wife — suggests the real-life price of the fluid line between performance and reality.
They also appreciate his keen sense of the sometimes fluid line between what the censors will deem acceptable for average Chinese to see on TV and what they will rule out as taboo.
The space between the films is filled with a fluid with unusual electrical and optical properties, so that, if an electrical field is established between the two thin films, the molecules of the fluid line up in such a way that the light-reflecting or light-transmitting properties of the assembly are radically changed.
"It's very fluid, [the line between] the music and the life, and I feel she's the only one who really accepted that, as far as the subjects of my songs go," she continues.
Nigeria's tensions are often painted as existing between the largely Muslim north and Christian-majority south, but a win that blurred those fluid lines points to a maturing democracy.
Friggin frozen fluid lines.
She had wonderful control and beautiful, fluid lines.
I was really taken with Massi's playful, fluid lines.
Remove the brake fluid lines.
