Abstract
An interferometric method is implemented in order to accurately assess the thermal fluctuations of a micro-cantilever sensor in liquid environments. The power spectrum density (PSD) of thermal fluctuations together with Sader’s model of the cantilever allow for the indirect measurement of the liquid viscosity with good accuracy. The good quality of the deflection signal and the characteristic low noise of the instrument allow for the detection and corrections of drawbacks due to both the cantilever shape irregularities and the uncertainties on the position of the laser spot at the fluctuating end of the cantilever. Variation of viscosity below 0.03 mPa·s was detected with the alternative to achieve measurements with a volume as low as 50 μL.
Highlights
The measurement of rheological properties, like fluid viscosity, is of common interest in several science fields, ranging from engineering to biology, with applications including process control [1,2,3,4]and fluids’ analysis for diseases diagnosis [5,6,7]
Force spectroscopy has emerged as a powerful tool to characterize the mechanics of soft materials at the nanoscale [10,11,12] and single molecule level [13]
The interferometric detection for cantilever deflection enables the measurement of the power spectrum density (PSD) of thermal fluctuations with a very high resolution and low noise
Summary
The measurement of rheological properties, like fluid viscosity, is of common interest in several science fields, ranging from engineering to biology, with applications including process control [1,2,3,4]and fluids’ analysis for diseases diagnosis [5,6,7]. AFM can be used in diverse modes to image either hard or soft surfaces in both dry and fluid environment [8,9]. Force spectroscopy has emerged as a powerful tool to characterize the mechanics of soft materials at the nanoscale [10,11,12] and single molecule level [13]. The selective adsorption of molecules onto the cantilever surface induces surface stresses, which leads to a deflection signal. Thermal noise detection is a less known and promising technique that has proven useful for the characterization of the cantilever mechanics [16] and its surrounding media [17,18]
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