= Screened sinusoidal surface-charge mode
{title2=$p=\sqrt{k^2+\kappa_D^2}$}
A lateral sinusoidal <surface charge density> with wave number $k$ in the <Debye–Hückel approximation> generates vertical exponential or hyperbolic functions with inverse length $p=\sqrt{k^2+\kappa_D^2}$. In a homogeneous electrolyte filling all space, an isolated sheet at $z_s$ produces $\phi=\sigma_k(x)e^{-p|z-z_s|}/(2\varepsilon p)$. Boundary conditions determine the normalization when the electrolyte occupies only a gap.
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