Pressure of thermal excitations in superfluid helium

We find the pressure, due to the thermal excitations of superfluid helium, at the interface with a solid. The separate contributions of phonons, R− rotons and R+ rotons are derived. The pressure due to R− rotons is shown to be negative and partially compensates the positive contribution of R+ rotons, so the total roton pressure is positive but several times less than the separate R− and R+ roton contributions. The pressure of the quasiparticle gas is shown to account for the fountain effect in He II. An experiment is proposed to observe the negative pressure due to R− rotons.