Optical Tweezers Flow Velocimetry (OTV)
To measure the ciliary flow with a high temporal resolution, we implement an optical-tweezers-based flow velocimetry (OTV). In this technique, a bead is trapped by a focused laser beam and reports the local flow velocity by its displacement from the laser focal point.
Essentially, the desired temporal resolution in measuring the flow (0.1 ms) is achieved by
employing back focal plane interferometry in monitoring the bead displacement Δx(t).
The left figure shows the typical experimental configuration (a), the Boundary Element Method computation of the flow field (b), and the comparison between the OTV measurement and the BEM computation (c).
It took 2 years (2015.09 - 2017.07) to get the technique to work. And we started to implement it thereafter. After a while, P. Dehnavi elaborated the technique in this paper Experiments in Fluids (2020) 61:202:
Optical tweezers‑based velocimetry: a method to measure microscale unsteady flows
The Unsteadiness in the Ciliary Flow:
Stokes Equations Do Not Show the Whole picture
Time resolution is not the only advantage of the OTV technique. The key is signal to noise ratio. The trapped bead undergoes confined Brownian motion, whose noise spectrum is a well known Lorentzian function. On this basis, once we have knowledge about the approximate frequency range of the ciliary beating, we can focus filter out all the irrelevant range.
This enhanced signal-to-noise ratio helps us to detect flow as low as ~2 µm/s at ~100 µm distance away from the cell. We first revealed that the flow from the cell is increasingly phase shifted.
The figure above shows how the measured signal agrees with the Stokes prediction close to the cell (a), in both amplitude and phase (c); and how the measured flow amplitude has become smaller than the prediction and is phase delayed (d).
We then characterized the flow's rate of spatial decay and the phase delay in different directions. Results show that the flow behaviors deviate fundamentally from what the Stokes equations predict but are well captured by the unsteady Stokes equations:
Detail of this work is now published in Physical Review Letters 122, 124502:
Is the Zero Reynolds Number Approximation Valid for Ciliary Flows?
A more systematic characterization of the flow around beating cilia of C. reinhardtii can now be found Journal of Fluid Mechanics, 915, A70:
Measurements of the unsteady flow field around beating cilia.
In the latter, you can see the transient flow field patterns around beating cilia (dt=~2ms) and hence how momentum (vorticity) propagates.
Hydrodynamics of the Fibrous Flagellar Hair
I am hopelessly attracted by the green algae C. reinhardtii. Its hydrodynamics is only a part of my research interest. With the OTV technique, we further characterize the structure-fluid interaction of a flagellar ultrastructure, mastigonemes.
Mastigonemes are like hairs protruding from flagella/cilia and there are multiple types of them. The C. reinhardtii mastigonemes have long been thought to enhance the swimming speed of this micro-organism. However, we refute this hypothesis after comparing the OTV flow measurements, numerical simulations, flagellar gait kinematics, and swimming kinematics of the cells with and without this ultrastructure. In the figure above, (a-c) are the TEM images of control groups that have mastigonemes, and (d) the mutant that doesn't.
Detail of this work is now published in Biophysical Journal:
Fibrous Flagellar Hairs of Chlamydomonas reinhardtii Do Not Enhance Swimming
Although the mastigonemes' missing seem to make no difference in flagellar propulsion, it does change the cells' swimming. Preliminary data show that the cells without mastigonemes undergo abrupt turnings more frequently.
Our speculation is that these structures are part of the sensor subunits, and are involved in the motion control. Some recent findings support this hypothesis.
Graphene & Superconductor Hybrid Devices
Before switching field to biophysics and quit my previous PhD, I design and fabricate different graphene nano-electronics.
I am quite proud of these Graphene & Superconductor hybrid devices we made 2013-2014. Graphene monolayers or bilayers deposited on doped silicon chips by mechanical exfoliation. Two graphene double quantum dots (DQDs) are fabricated within the same alignment array and a reflection line resonator (RLR) is fabricated to connect the gates of the DQDs.
With this architecture, we successfully characterized the decoherence time (T2) of the charge states in graphene DQDs and see the distant coupling between two states mediated by the photons in the resonator.
The results are published in Nano Letters 15, 10, 6620–6625:
Coupling Two Distant Double Quantum Dots with a Microwave Resonator
and in Physical Review Letters 115, 126804:
and in Chinese Physics Letters 33(04):047301 :
Multiplexing Read-Out of Charge Qubits by a Superconducting Resonator
I am in charge of the sample design, fabrication, part of the measurement and analysis. I am very proud to conceive and initiate the project together with my friends and colleagues. Starting from this project, I realized how much I enjoy close collaborations and how much I enjoy making "whimsical" ideas come true.
Fabrication and Characterization of Grpahene Quantum Dots
Locked in the cleanroom, sang in the huge noise of those mechanical pumps and molecular pumps. Nothing worked and tens of hours were used in making samples that later proved to be trash. This is how I remembered the year 2011 to 2013.
I made it in the end and the production of graphene nano-electronics is no longer a problem. I implement a modified measurement scheme of the electron temperature and realized that it can also tell the tunnelling rate between the quantum dots.
The work is published in Scientific Reports 3, 3175:
Tuning inter-dot tunnel coupling of an etched graphene double quantum dot by adjacent metal gates
This is my first-ever first-authored scientific publication and is also the first time me going through the whole process from submission to final textual check.