Critical adsorption in the undersaturated regime

The interaction between a near-critical binary liquid and a solid substrate can be modeled by an effective surface field h₁. Critical adsorption experiments to date have been done in the limit of strong field, but we have been able to produce intermediate-strength fields by chemically modifying a surface. We present here the first experimental demonstration of critical adsorption in this undersaturated regime. Our reflectivity data indicate that critical adsorption can remain undersaturated even within 3 mK of Tc---a surprising result difficult to reconcile with current expectations of the behavior of the critical scaling function.