Pulsed Laser Viscometer: Accurate High-Speed Sensing Technique for Viscosity and Surface Tension of Liquids

The pulsed laser viscometer (PLV) measures viscosity and surface tension using nanoscale liquid surface deformation generated by two-beam interference of a pulsed heating laser and detects damping oscillation from the first-order diffracted intensity of a probing laser. PLV features (1) non-contact and in situ measurement, (2) high spatial resolution of 10 to 100 µm, (3) high time resolution of microseconds to milliseconds, (4) minute sample volume of microliters to milliliters, and (5) a wide viscosity range of 0.1 to 10 ⁴ mPa·s. We established the PLV theory by solving the Navier‒Stokes‒Thermoelastic equation for volumetric heating of a liquid by a weakly absorbed pulsed laser. A new PLV experimental apparatus was developed using a pulsed YAG laser with a wavelength of 1064 nm as the heating laser, and experiments used six liquid samples (acetone, toluene, water, ethanol, 2-propanol, and 1-hexanol) at room temperature to verify that the instrument operates according to the theory. Measured viscosities agree with the literature values with an average deviation of 3.2%. Measured surface tension agrees with the literature values with an average deviation of 6.2%. Uncertainties are estimated at 2% for viscosity and 3% for surface tension. The temperature rise of the sample during the experiment was less than 0.01 K. Furthermore, eight other thermophysical properties required in the PLV theory (such as absorption coefficient and volume expansion coefficient) only shift the signal waveform vertically, and the influence of viscosity and surface tension on the curve-fitting results remains dominant.