2008 Nanoday Poster
Title: Thermal Bubble Nucleation in Nanochannels: Simulations and Strategies for Nanobubble Nucleation and Sensing
, Manoj Sridhar, Dongyan Xu, Anthony Hmelo, Deyu Li and Leonard Feldman
Abstract: With continuing progress in the state of the art of nanofabrication it is now possible to conceive devices that may enable the experimental sensing of bubble nucleation in nanochannels, and the direct measurement of the nucleation rate in water and other fluids. In the present study the authors present the results of molecular dynamics simulations of thermal bubble nucleation in nano-confined argon and water systems using an isothermal-isobaric (NPT) ensemble to determine the conditions under which nanobubble nucleation may be expected. No bubbles were observed for either system under an external pressure of 0.01 - 0.1 MPa, even for temperatures much higher than the boiling temperature of the respective liquids at 0.1 MPa. The density of the nano-confined fluids at constant temperature is observed to be almost independent of external pressure on the system in the simulated pressure range, suggesting that the nano-confined liquids behave like liquids with low compressibility even at temperatures close to their superheat limit. To explain these observations, we hypothesize that bubble nucleation induces a pressure disturbance, which travels to the channel wall and reflects back to the nucleation site suppressing bubble nucleation as the characteristic pressure wave travel time is much shorter than the nucleation time. Our results suggest limits on the nanochannel length scale and conditions under which nanobubble nucleation can be expected.
The experimental sensing of bubble nucleation in a nanochannel reactor requires an advanced detection scheme. We report on the detailed characterization of an ultrasensitive fluidic device that has been used to detect the translocation of small particles through a sensing microchannel. The device connects a ﬂuidic circuit to the gate of a metal-oxide-semiconductor ﬁeld-effect transistor (MOSFET) and detects particles by monitoring the MOSFET drain current modulation instead of the modulation in the ionic current through the sensing channel. The minimum volume ratio of the particle to the sensing channel detected is 0.006%, which is about ten times smaller than the lowest detected volume ratio previously reported in the literature. This volume ratio is detected at a noise level of about 0.6% of the baseline MOSFET drain current, clearly showing the ampliﬁcation effects from the ﬂuidic circuits and the MOSFETs. We characterized the device sensitivity as a function of the MOSFET gate potential and show that its sensitivity is higher when the MOSFET is operating below its threshold gate voltage than when it is operating above the threshold voltage. In addition, we demonstrated that the device sensitivity linearly increases with the applied electrical bias across the ﬂuidic circuit. We demonstrate the application of the device concept as a particle sensor for polystyrene and glass beads on a variety of micro/nano length scales.
2008 Nanoday Poster
Title: Measuring Depth-dependent Defect Concentrations in Semiconductors using Coherent Acoustic Phonon Waves, A. Steigerwald, J. Gregory, J. Qi, Y. Xu, A. Hmelo and N. Tolk
Abstract: Determining depth profiles of defect concentrations is a difficult challenge in materials characterization. Currently there are no robust methods to achieve defect concentration profiles over a broad depth range. Available techniques are often of limited use due to material specificity or shallow depth limits, can be destructive in nature, and tend to focus on composition profiles while ignoring point defects such as vacancies or self-interstitials. Using ultra-fast time-resolved pump-probe techniques, we demonstrate that optically excited coherent acoustic phonon (CAP) waves may be employed to obtain point defect concentration depth profiles in semiconductors for concentrations spanning almost four orders of magnitude. This non-invasive, non-destructive method has a depth range in excess of tens of microns and can be used for a variety of materials containing different defect types, offering a novel and readily accessible route to defect depth profiling.
Title: An Ultrasensitive MOSFET-Based Coulter Counter, M. Sridhar, Y. Kang, D. Xu, A.B. Hmelo, L.C. Feldman, D. Li and D. Li
2006 Nanoday Poster
Title: Electro-osmotic Flow Through Surface Modified Nanochannels, M. Sridhar, D. Li, A.B. Hmelo and L.C. Feldman
2005 Nanoday Poster
Title: Colloidal Jamming in Nanoporous Membranes, M. Sridhar, S. Hasan, S.K. Vajandar, A.B. Hmelo, L.C. Feldman, D. Li, S.J. Rosenthal and R.S. Foote