Figure 1

Principle of method. Panel a: The ability of a meniscus to act as a lens. The surface of water is nearly flat in polystyrene vessels such as disposable cuvettes and microtitre plates, but a meniscus is produced when the surface tension is lowered (from 72 to ∼35 mN/m) by the addition of detergent. The resulting curved surface evident in the side view allows the liquid to act as a demagnifying lens. This phenomenon is demonstrated in the photograph of a grid of 20-mm squares taken through the samples in a 96-well plate positioned 100 mm above the grid. Panel b: The extent of the lens-like effect of a solution meniscus can be quantified by measuring the relative intensity of a light beam passed through the liquid. When the surface is flat (i), refractive and reflective light losses are minimal. When the surface is a curved meniscus (ii), some light is lost due to these processes and does not reach the detector. This effect can be enhanced by passing the light beam through the sample near the edge of the well (iii), where the surface curvature is greater. The light intensity varies as a function of the contact angle (θ) of the liquid and the side of the vessel, which is in turn related to surface tension.