Untying molecular friction knots

Motivated by recent advances in single molecule manipulation techniques that enabled several groups to tie knots in individual polymer strands and to monitor their dynamics, we have used computer simulations to study "friction knots" joining a pair of polymer strands. The key property of a friction knot splicing two ropes is that it becomes jammed when the ropes are pulled apart. In contrast, molecular friction knots eventually become undone by thermal motion. We show that depending on the knot type and on the polymer structure, a friction knot between polymer strands can be strong (the time t the knot stays tied increases with the force F applied to separate the strands) or weak (t decreases with increasing F). We further present a simple model explaining these behaviors.