Optical Fiberglass Tip for Cantilever Based Optical Interfacial Force Microscopy in Liquid
Application of the cantilever-based optical interfacial force microscope (COIFM) in an aqueous environment is important in the understanding of how various interactions depend on the distance between two surfaces, such as colloidal particles. However, the COIFM had difficulty probing those interactions in a liquid environment because it uses an electrical signal for force-feedback. To solve this issue, an isolation of the electrical feedback was needed. This was accomplished by using an optical fiberglass tip that was long enough to penetrate the fluid while the electrode of the cantilever was still unexposed to the aqueous environment. The fabrication of the optical tip used a chemical etching method of submersion in hydrofluoric acid that sharpened the tip. Through the combined use of a micromanipulation stage and an optical microscope, the fabricated tip was then brought and attached to the end of the COIFM force sensor using a UV epoxy. The attached optical tip was characterized by imaging a standard two-dimensional grating structure with 10 mm periodicity in air. The performance of the COIFM force sensor with the optical fiber tip was tested by measuring the force as a function of distance between the tip and an oxidized silicon surface in water. The force-distance curve showed that the force increased rapidly as the tip approached the sample surface. This result is consistent with previous observations of hydration force between two hydrophilic surfaces studied by other methods. This consistency suggests that the COIFM combined with the optical fiberglass tip is capable of measuring force in a liquid environment.