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Chen, Kejia

Publications and source records attributed to Chen, Kejia.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

DNA molecules deviate from shortest trajectory when driven through hydrogel

Dynamic fluorescence-based single-molecule imaging of $λ$-DNA molecules driven through agarose hydrogels by DC electric fields reveals that passage through the hydrogel (98.5% water content) induces mobility orthogonal to the external field. Tortuous paths followed by the DNA molecules, which are heavily entangled in the hydrogel mesh as their contour length is nearly 100 times the hydrogel mesh size of 200 nm, cause them to appear to diffuse orthogonal to the driving force. The higher the driving field, from 2 to 16 V/cm, the higher the off-axis dispersion is, over the same time interval. In this work we measure the off-axis displacement distribution over 3 orders of magnitude of probability density and find a master curve after normalizing for time (t) elapsed, but the power of time for normalizing increases with the external field, from t 0.25 to t 0.6 with increasing field. Comparing trajectories over the same distance traveled in the electric field direction, we observe whereas for the highest field strengths DNA molecules come closest to taking the shortest trajectory between two points in space, deviations from the shortest trajectory grow larger and larger (up to 40% larger) as one approaches the case of small yet finite external field strength.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Memoryless self-reinforcing directionality in endosomal active transport within living cells

In contrast to Brownian transport, the active motility of microbes, cells, animals and even humans often follows another random process known as truncated Levy walk 1,2 . These stochastic motions are characterized by clustered small steps and intermittent longer jumps that often extend towards the size of the entire system. As there are repeated suggestions, although disagreement, that Levy walks have functional advantages over Brownian motion in random searching and transport kinetics 3-8 , their intentional engineering into active materials could be useful. Here, we show experimentally in the classic active matter system of intracellular trafficking 9-15 that Brownian-like steps self-organize into truncated Levy walks through an apparent time-independent positive feedback such that directional persistence increases with the distance traveled persistently. A molecular model that allows the maximum output of the active propelling forces to fluctuate slowly fits the experiments quantitatively. Furthermore, our findings offer design principles for programming efficient transport in active materials.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Extending particle tracking capability with Delaunay triangulation

Here, particle tracking, the analysis of individual moving elements in time series of microscopic images, enables burgeoning new applications, but there is need to better resolve conformation and dynamics. Here we describe the advantages of Delaunay triangulation to extend the capabilities of particle tracking in three areas: (1) discriminating irregularly shaped objects, which allows one to track items other than point features; (2) combining time and space to better connect missing frames in trajectories; and (3) identifying shape backbone. To demonstrate the method, specific examples are given, involving analyzing the time-dependent molecular conformations of actin filaments and λ-DNA. The main limitation of this method, shared by all other clustering techniques, is the difficulty to separate objects when they are very close. This can be mitigated by inspecting locally to remove edges that are longer than their neighbors and also edges that link two objects, using methods described here, so that the combination of Delaunay triangulation with edge removal can be robustly applied to processing large data sets. As common software packages, both commercial and open source, can construct Delaunay triangulation on command, the methods described in this paper are both computationally efficient and easy to implement.

36 MATERIALS SCIENCE↗