  The study of microsystems and the development of nanotechnologies require new
techniques to measure piconewton and femtonewton forces at microscopic and
nanoscopic scales. Amongst the challenges, there is the need to deal with the
ineluctable thermal noise, which, in the typical experimental situation of a
spatial diffusion gradient, causes a spurious drift. This leads to a correction
term when forces are estimated from drift measurements [Phys. Rev. Lett. 104,
170602 (2010)]. Here, we provide a systematic study of such effect comparing
the forces acting on various Brownian particles derived from equilibrium
distribution and drift measurements. We discuss the physical origin of the
correction term, its dependence on wall distance, particle radius, and its
relation to the convention used to solve the respective stochastic integrals.
Such correction term becomes more significant for smaller particles and is
predicted to be in the order of several piconewtons for particles the size of a
biomolecule.
