In the single rent-to-buy decision problem, without a priori knowledge of the amount of time a resource will be used we need to decide when to buy the resource, given that we can rent the resource for $1 per unit time or buy it once and for all for $c. In this paper we study algorithms that make a sequence of single rent-to-buy decisions, using the assumption that the resource use times are independently drawn from an unknown probability distribution. Our study of this rent-to-buy problem is motivated by important systems applications, specifically, problems arising from deciding when to spindown disks to conserve energy in mobile computers [4], [13], [15], thread blocking decisions during lock acquisition in multiprocessor applications [7], and virtual circuit holding times in IP-over-ATM networks [11], [19].We develop a provably optimal and computationally efficient algorithm for the rent-to-buy problem. Our algorithm uses O( √ t) time and space, and its expected cost for the tth resource use converges to optimal as O( log t/t), for any bounded probability distribution on the resource use times. Alternatively, using O(1) time and space, the algorithm almost converges to optimal.We describe the experimental results for the application of our algorithm to one of the motivating systems problems: the question of when to spindown a disk to save power in a mobile computer. Simulations using disk access traces obtained from an HP workstation environment suggest that our algorithm yields significantly improved power/response time performance over the nonadaptive 2-competitive algorithm which is optimal in the worst-case competitive analysis model.

Fabrication of covalent organic framework (COF) membranes for molecular transport has excited highly pragmatic interest as a low energy and cost-effective route for molecular separations. However, currently, most COF membranes are assembled via a one-step procedure in liquid phase(s) by concurrent polymerization and crystallization, which are often accompanied by a loosely packed and less ordered structure. Herein, we propose a two-step procedure via a phase switching strategy, which decouples the polymerization process and the crystallization process to assemble compact and highly crystalline COF membranes. In the pre-assembly step, the mixed monomer solution is casted into a pristine membrane in the liquid phase, along with the completion of polymerization process. In the assembly step, the pristine membrane is transformed into a COF membrane in the vapour phase of solvent and catalyst, along with the completion of crystallization process. Owing to the compact and highly crystalline structure, the resultant COF membranes exhibit an unprecedented permeance (water ≈ 403 L m−2 bar−1 h−1 and acetonitrile ≈ 519 L m−2 bar−1 h−1). Our two-step procedure via phase switching strategy can open up a new avenue to the fabrication of advanced organic crystalline microporous membranes.

Engineering surface chemistry to precisely control interfacial interactions is crucial for fabricating superior antifouling coatings and separation membranes. Here, we present a hydrophobic chain engineering strategy to regulate membrane surface at a molecular scale. Hydrophilic phytic acid and hydrophobic perfluorocarboxylic acids are sequentially assembled on a graphene oxide membrane to form an amphiphilic surface. The surface energy is reduced by the introduction of the perfluoroalkyl chains while the surface hydration can be tuned by changing the hydrophobic chain length, thus synergistically optimizing both fouling-resistance and fouling-release properties. It is found that the surface hydration capacity changes nonlinearly as the perfluoroalkyl chain length increases from C4 to C10, reaching the highest at C6 as a result of the more uniform water orientation as demonstrated by molecular dynamics simulations. The as-prepared membrane exhibits superior antifouling efficacy (flux decline ratio <10%, flux recovery ratio ~100%) even at high permeance (~620 L m−2 h−1 bar−1) for oil-water separation.

Structural diversity obtained through two-dimensional molecular self-assembly induced by the chain length effect has gained immense attention, not only because of its significance in crystal engineering but also for its potential application in nanoscience and nanotechnology. Three kinds of fluorenone derivative, named F-CC, F-CC, and F-CC, were synthesized and used for systematic exploration of their crystalline difference. At first, scanning electron microscopy and X-ray powder diffraction were performed to investigate their differences in morphology and three-dimensional crystal structure. Then scanning tunneling microscopy experiments were conducted to compare the self-assembled monolayers. Moreover, different solvents were used to repeatedly investigate the occurrence of structural diversity. F-CC could not self-assemble into a stable monolayer on the graphite surface under ambient conditions due to its weak molecule-substrate interaction. F-CC was observed to self-assemble into twist, plier-like, octamer-curve, and random structures in 1-octanoic acid, 1-phenyloctane, n-tetradecane, and dichloromethane, respectively. However, when the same solvents were used and at similar concentrations, the F-CC molecules were arranged into interval, mixed, linear, and plier-like configurations. These self-assembled nanopatterns formed under the driving forces of dipole-dipole interactions, hydrogen bonds, and chain-chain, molecule-substrate, and molecule-solvent van der Waals interactions. Furthermore, from the viewpoint of thermal analysis, differential scanning calorimetry, as well as polarized optical microscopy, was performed to further elucidate the difference between these three compounds in the solid and liquid crystal states. The present system is believed to provide understanding of how the chain length effect induces different crystalline properties, and to open up the possibility of fabricating diverse self-assembled networks for crystal engineering.