Zhengdong Cheng
Post-Doctoral Fellow, Dave
Weitz group, Harvard University
Current
Research
Discotic
colloids
Colloids come in a variety of shapes, including
sphere (the well-known hard sphere PMMA, or charge sphere PS, Silica), rod (TMV, fd), or
palate (Gibbsite). Here we use drop break-off in a co-flowing stream to
produce uniform wax coins. These microdisks are made by crystallizing a-eicosene
(CH3-(CH2)17-CH=CH2) aqueous emulsion particles. The right
circular cylindrical disks arises when the cooled wax enters a lamellar liquid
crystalline rotator phase that occurs at 29.8oC. The lamellar planes
lie parallel to the flat surface, and the in-plane orientational disorder of
the wax molecules facilitates an in-plane isotropic surface energy that permits
a circular perimeter. The microdisks have radii ranging from 0.2 mm to
10 mm, and aspect ratio from 2 to 10 with 5 typically.
Using these disks, we built the smallest
lighthouse in the world. Circularly polarized light continuously powers these
lighthouses.
In a optical trap, pushing the disk again the
glass slide, a periodic motion would be excited. This is the first observation
of “Pioncare-Andronov-Hopf”
bifurcation in laser tweezers.
Figure 1
Frequency doubling to Chaos
The following figure presents evidence of
frequency doubling to chaos in water droplets production in microfluidic
devices with flow focusing configuration. At water flow rate 265 ml/hr,
the transition is the bump-sprout transition of the water tip. A series
frequency doubling and halving take place in the range from 100 ml/hr
to 60 ml/hr due to the oscillation of the water-tip, leading
eventually to the chaotic behavior. Research efforts are put seeking control of
this dynamics bifurcation by surfactants, electric field etc.
CV
Contact information:
Division of
Engineering and Applied Sciences/Department of Physics
ESL, Office
202 Phone: (617) 384-5929, Cell:
510-590-1194