Efficient Reduction of Casimir Forces by Self-assembled Bio-molecular Thin Films

Abstract

Casimir forces, related to London-van der Waals forces, arise if the spectrum of electromagnetic fluctuations is restricted by boundaries. There is great interest both from fundamental science and technical applications to control these forces on the nano scale. Scientifically, the Casimir effect being the only known quantum vacuum effect manifesting between macroscopic objects, allows to investigate the poorly known physics of the vacuum. In this work, we experimentally investigate the influence of self-assembled molecular bio and organic thin films on the Casimir force between a plate and a sphere. We find that molecular thin films, despite being a mere few nanometers thick, reduce the Casimir force by up to 14%. To identify the molecular characteristics leading to this reduction, five different bio-molecular films with varying chemical and physical properties were investigated. Spectroscopic data reveal a broad absorption band whose presence can be attributed to the mixing of electronic states of the underlying gold layer and those of the molecular film due to charge rearrangement in the process of self-assembly. Using Lifshitz theory we calculate that the observed change in the Casimir force is consistent with the appearance of the new absorption band due to the formation of molecular layers. The desired Casimir force reduction can be tuned by stacking several monolayers, using a simple self-assembly technique in a solution. The molecules - each a few nanometers long - can penetrate small cavities and holes, and cover any surface with high efficiency. This process seems compatible with current methods in the production of micro-electromechanical systems (MEMS), which cannot be miniaturized beyond a certain size due to `stiction' caused by the Casimir effect. Our approach could therefore readily enable further miniaturization of these devices.

Force Metrology with Plane Parallel Plates: Final Design Review and Outlook

“…Apart from patch potentials, two opposing surfaces differ in their absolute potential even if grounded together due to contact potentials. To cancel these, we use an active homodyne compensation method that was successfully applied in recent measurements of Casimir forces [122][123][124] and in the proof of principle for CANNEX [103]. The method is similar to amplitude modulation Kelvin probe force microscopy (see below), and relies on a small electrostatic excitation, v AC (t) = V ac sin ω AC t, being applied to the lower plate, resulting in signals at frequencies ω AC and 2ω AC , whose amplitudes are measured using a lock-in amplifier.…”

“…The sensor plate and shroud (see Figure 1) of the force sensing setup are replaced by a u-shaped structure (green) carrying an AFM cantilever. The optical fiber, which is otherwise used for measurements on the sensor plate, is remounted at an angle to detect the movement of the cantilever interferometrically, in a similar way as demonstrated previously [124,126]. In order to align the fiber with the cantilever tip in situ, we use a stack of horizontal stick-slip translators.…”