I'm not sure that you can find a physical thing that is the life force. I believe that a life force exists.
I've heard of cases where a married person will be so heart broken by the death of his or her spouse that they loose the will to live. There are also cases where a man who has no hobby doesn't really have much to live for retires and he dies a few years later. Maybe its something else that made them die, but even if it did, there might be something else that chose to accept that fate. (I don't mean to disregard the state of the body. A beheaded medivial criminal might have desparately wanted to live, but it just doesn't work that way.) Sometimes a person wants to live so much even with so many things and somehow they don't die. I think that it is the result of some kind of life force in each of us. That is something that is so closely tied to the physical realm, at the same time exists outside of it, like the mind-body connection.
In a way, all creatures probably have some kind of life force like that. If you try to smash two bacteria together, (I am assuming that they are alive) you are trying to smash to life forces then together. They cannot become on. At least one entity will die in the process. (I hope I don't sound totally wacked.)