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Thermal membrane undulations generate Helfrich repulsion from a nearby particle. For a sphere of radius $R$ tightly enclosed by a high-tension membrane,

Helfrich repulsion is the entropic interaction produced when neighboring membranes suppress each other's thermal undulations. Its free energy per area scales as $(k_{B}T{)}^2/(k_c{d}^2)$.

A membrane unbinding transition sends the equilibrium layer spacing to infinity. In a dilute lamellar stack, competition between a quadratic virial attraction and cubic Helfrich repulsion gives a continuous transition with spacing exponent one.

In the high-tension, narrow-gap regime, the almost spherical membrane has area $4\pi (R+\delta {)}^2$. Its spherical bending energy $8\pi k_c$ is independent of radius, while Helfrich repulsion over area $4\pi R^2$ costsUp to terms independent of $\delta$, a suitable free energy is consequentlyIts stationary point satisfiesUsing $\delta /R\ll 1$ gives the fluctuation-supported membrane--particle gapThe inverse-square entropic repulsion prevents contact, while membrane tension limits the area gained by opening the gap.

At large separation, the finite-thickness van der Waals combination has the expansionThus the attraction governed by the Hamaker constant decays asand the screened electrostatic term decays exponentially on the Debye–Hückel screening length. The Helfrich repulsion, however, decays only asConsequently the total interaction approaches its unbound value zero from above as $d\to \infty$.

A finite bound minimum must have nonpositive energy to beat the state at infinity. Because the large-$d$ interaction is positive, such a minimum cannot move continuously to infinity while remaining globally stable. At the transition it instead becomes degenerate with the $d=\infty$ state at a finite spacing and then loses global stability. The equilibrium spacing therefore jumps from finite $d_{\ast }$ to infinity, making this a discontinuous, first-order unbinding transition.