  Here we investigate how the positions of a condensed phase can be controlled
by using concentration gradients of a regulator that influences phase
separation. We consider a mean field model of a ternary mixture where a
concentration gradient of a regulator is imposed by an external potential. We
show that novel first order phase transition exists at which the position of
the condensed phase switches in a discontinuous manner. This mechanism could
have implications for the spatial organization of biological cells and provides
a control mechanism for droplets in microfluidic systems.
