Research

Electrostatic charging of granular materials

Particle interactions in charged granular streams:
We observe individual collide-and-capture events between charged13xsr8 submillimeter particles, including Kepler-like orbits in granular streams. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation. Screen Shot 2016-05-10 at 10.11.33 PMMoreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids.

 

Size-dependent tribocharging in insulating grains:
Screen Shot 2016-01-22 at 11.59.29 AMObservations of flowing granular matter have suggested that same-material tribocharging depends on particle size, typically rendering large grains positive and small ones negative. Models assuming the transfer of trapped electrons can account for this trend, but have not been validated. Tracking individual grains in an electric field, we show quantitatively that charge is transferred based on size between materially identical grains. However, the surface density of trapped electrons, measured independently by thermoluminescence techniques, is orders of magnitude too small to account for the scale of charge transferred. This reveals that trapped electrons are not a necessary ingredient for same-material tribocharging.

 

Using acoustic levitation to study tribocharging of sub-millimeter particles

We present a new method that allows us to measure 13zk6othe change of net charge on individual sub-millimeter particles with high precision. We do that by acoustically suspending the particle in air and tracking its motion in response to applied ac and dc electric fields with a high-speed camera.  We investigate tribocharging of the particle by temporarily switching off the levitation, thereby creating a controlled sequence of collisions with a target surface.  This technique opens up new possibilities to study the charge transfer process during a single contact for tribocharging of granular materials.

 

Thermal transport in nanowires & carbon nanotubes

Divergent thermal conductivity in ultralong carbon nanotubes:
Screen Shot 2016-01-19 at 5.24.56 PMLow-dimensional materials could be non-Fourier thermal conductors displaying divergent thermal conductivity with increasing lengths, in ways inconceivable in any bulk materials. However, previous theoretical or experimental investigations were plagued with many finite-size effects, rendering the results either indirect or inconclusive. We observe that the thermal conductivity of single-wall carbon nanotubes continuously increase with their lengths over 1mm, reaching at least 8640 W/m-K at room temperature. Remarkably, the non-Fourier thermal conduction persists even with the presence of defects, isotopic disorders, impurities, and surface absorbates. The finding would open new regimes for wave engineering of heat as well as manipulating phonons at macroscopic scales.

 

One-dimensional heat transfer under extreme bending strain:
Screen Shot 2016-01-22 at 10.11.08 AMWe utilize the superior mechanical strength and the high thermal conductivity of single-wall carbon nanotubes (SWCNTs) to investigate the heat transfer phenomena at a previously inaccessible experimental regime. Surprisingly, even when the SWCNTs are bent far beyond their critical angles and curvatures, their thermal conductivities remain intact under cyclic bending.

 

Thermal conduction of nanowires/nanotubes under bending:
nanowire
We observe universal robustness of thermal conduction against bending strain for various one-dimensional materials (Ag, ZnO, Ga2O3, Ge, Si, GaN, and SiGe nanowires, single-wall and multi-wall carbon nanotubes). The surprising robustness of thermal conduction is found to be independent of lattice structures, electronic band gaps (0~5eV), thermal conductivities (8~2000 W/m-K), mean free paths (5~8300nm), bending angles (0~242), bending curvatures (0~45μm-1), and surface roughness; and it sustains until the samples fracture. On the other hand, electronic properties of these materials do not display similar phenomena. The finding will help to improve nanoelectromechanical systems operating at extreme conditions.

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