Abstract-Wireless sensor networks have been widely used for surveillance in harsh environments. In many such applications, the environmental data are continuously sensed, and data collection by a server is only performed occasionally. Hence, the sensor nodes have to temporarily store the data, and provide easy and on-hand access for the most updated data when the server approaches. Given the expensive server-to-sensor communications, the large amount of sensors and the limited storage space at each tiny sensor, continuous data collection becomes a challenging problem.In this paper, we present partial network coding (PNC) as a generic tool for the above applications. PNC generalizes the existing network coding (NC) paradigm, an elegant solution for ubiquitous data distribution and collection. Yet, PNC enables efficient storage replacement for continuous data, which is a major deficiency of the conventional NC. We prove that the performance of PNC is quite close to NC, except for a sublinear overhead on storage and communications. We then address a set of practical concerns toward PNC-based continuous data collection in sensor networks. Its feasibility and superiority are further demonstrated through simulation results.

The air/liquid interface of aqueous methanol solutions, one of the earliest benchmark liquid interfaces ever studied by surface-selective sum frequency generation vibrational spectroscopy (SFG-VS), is known to display a turnover of SFG-VS intensities at the intermediate methanol concentrations, the origin of which has been under debate for the past 25 years. In this work, we discover that the hyperpolarizability of methanol, which was previously assumed to be a constant, in fact strongly depends on the bulk mole fractions of the methanol molecules. Furthermore, such concentrationdependence of the methanol hyperpolarizability is shown to play a critical role in the nonmonotonic changes of the methanol SFG-VS intensities as a function of the bulk mole fractions. By comparing the results from the molecular dynamics simulations and the newly interpreted SFG-VS spectra, a consistent surface model is obtained, showing that the average tilt angles of the asymmetrically oriented interfacial methanol molecules that actually contribute to the net SFG-VS signals are independent of the concentrations.

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Increasing horizontal and face-on orientations of the active molecules in organic solar cells is critical to improve light absorption, charge mobility, and exciton diffusivity for high photovoltaic efficiency. However, how to effectively control the molecular orientations is challenging and still unclear.Here, the molecular self-assembly and orientation formation are elucidated for a series of acceptor-donor-acceptor (A-D-A) small-molecule acceptors (SMAs) by atomistic molecular dynamics simulations. The results indicate that the A-D-A molecules tend to assemble via the end-group π-π stacking at the liquid-vapor interfaces during solvent evaporation, which is beneficial to enhance horizontal and face-on orientations. Remarkably, the order of both horizontal and face-on orientations in the films shows a good linear correlation with the number of π-π stacking among all the A-D-A SMAs. The π-π stacking is found to be cooperatively controlled by both the end-group π-π interaction and side-chain steric hindrance. This work provides the rationalization for the different molecular orientations in a wide range of A-D-A SMAs and is useful to improve the horizontal and face-on orientations for high-performance organic photovoltaics.