This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordThis paper presents a new methodology employing a super-twisting sliding mode observer to reconstruct un-measureable atomic-forces at nano-Newton precision in a Vertically Oriented Probe Microscope (VOPM). The VOPM senses the deflection of a vertically oriented cantilever, caused by shear-force interaction with a confined water layer above the sample-substrate. The paper describes the development of a model and the subsequent experimental process for computing its parameters. This forms the basis for the design of a super-twisting observer to estimate the unknown shear-forces. The reconstructed force can be decomposed into elastic and viscous components ,which are important in biological research.Engineering and Physical Sciences Research Council (EPSRC
