Il Lee

Note: Precision viscosity measurement using suspended microchannel resonators

We report the characterization of a suspended microchannel resonator (SMR) for viscosity measurements in a low viscosity regime (<10 mPa s) using two measurement schemes. First, the quality factor (Q-factor) of the SMR was characterized with glycerol-water mixtures. The measured Q-factor at 20 °C exhibits a bilinear behavior with the sensitivity of 1281 (mPa s)(-1) for a lower (1-4 mPa s) and 355 (mPa s)(-1) for a higher viscosity range (4-8 mPa s), respectively. The second scheme is the vibration amplitude monitoring of the SMR running in a closed loop feedback. When compared in terms of the measurement time, the amplitude-based measurement takes only 0.1 ~ 1 ms while the Q-factor-based measurement takes ~30 s. However, the viscosity resolution of the Q-factor-based measurement is at least three times better than the amplitude-based measurement. By comparing the Q-factors of heavy water and 9.65 wt.% glycerol-water mixture that have very similar viscosities but different densities, we confirmed that the SMR can measure the dynamic viscosity without the density correction. The obtained results demonstrate that the SMR can measure the fluid viscosity with high precision and even real-time monitoring of the viscosity change is possible with the amplitude-based measurement scheme.

Facile Phase Transition Measurements for Nanogram Level Liquid Samples Using Suspended Microchannel Resonators

We investigated phase transitions of a PEO-PPO-PEO triblock copolymer and n-heptadecane using a suspended microchannel resonator (SMR). After filling the microchannel of the SMR with each sample in liquid state, changes in the resonance frequency of the SMR were measured as a function of temperature, and then converted into changes in the density of each sample. As temperature increases, PEO-PPO-PEO unimers aggregate and form micelles (unimer-micelle transition), so the density of the polymer decreases and the resonance frequency of the SMR increases. As temperature decreases, n-heptadecane undergoes liquid to rotator phase (liquid-rotator transition), which increases the sample density and decreases the resonance frequency of the SMR. In addition, the liquid-rotator transition of n-heptadecane exhibits a sudden change in the quality factor of the SMR.

Fabrication and characterization of all hydrogel cantilevers for atomic force microscopy applications

This paper reports a novel method for fabricating hydrogel based microcantilevers by using dynamic mask lithography. A hydrogel, polyethyleneglycol diacrylate (PEGDA), was introduced between two parallel polydimethylsiloxane (PDMS) guides then cured with ultra-violet (UV) exposure to intended shape and size defined by the dynamic mask; an image sent from a PC to a liquid crystal display projector. One PDMS guide has an embedded glass piece which serves as a handle for the microcantilever and the other guide is with or without an inverted pyramid tip mold to fabricate tip-integrated or tipless microcantilevers, respectively. After fabricated hydrogel microcantilevers were thoroughly characterized by using a stylus profilometer and an atomic force microscope (AFM), they were employed for both contact and non-contact mode AFM imaging. In case of non-contact mode, the imaging performance of hydrogel AFM cantilevers was comparable to that of commercial silicon AFM cantilevers.

Quality factor and vibration amplitude based viscosity measurements using suspended microchannel resonators

In this paper, we report precision viscosity measurements using suspended microchannel resonators (SMRs). Two different methods are developed and applied to glycerol-water binary mixtures having various weight ratios. First, open loop frequency sweep and harmonic oscillator fitting is employed to extract the quality factor (Q-factor) of an SMR. Second, closed loop feedback operation is employed while the vibration amplitude is continuously monitored. Both the Q-factor and vibration amplitude increase with increasing viscosity for the tested SMR and they agree well after rescaling. Viscosity measurements using the Q-factor change are compared with measurements using a bench-top tuning fork viscometer. While the bench-top tuning fork viscometer measures static viscosity thus always requires density correction, the SMR Q-factor measurement provides dynamic viscosity directly. In addition, SMR measurements require sample volume of ~50 μL while the bench-top viscometer requires ~10 mL per each measurement.