Extensions of the sequential stochastic assignment problem

The sequential stochastic assignment problem (SSAP) allocates N workers to N IID sequentially arriving tasks so as to maximize the expected total reward. This paper studies two extensions of the SSAP. The first one assumes that the values of any two consecutive tasks are dependent on each other while the exact number of tasks to arrive is unknown until after the final arrival. The second extension generalizes the first one by assuming that the number of workers is also random. Optimal assignment policies for both problems are derived and proven to have the same threshold structure as the optimal policy of the SSAP.     

Ni-based catalysts for plasma-assisted CO2 methanation

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Stability analysis of parallel server systems under longest queue first

We consider the stability of parallel server systems under the longest queue first (LQF) rule. We show that when the underlying graph of a parallel server system is a tree, the standard nominal traffic condition is sufficient for the stability of that system under LQF when interarrival and service times have general distributions. Then we consider a special parallel server system, which is known as the X-model, whose underlying graph is not a tree. We provide additional “drift” conditions for the stability and transience of these queueing systems with exponential interarrival and service times. Drift conditions depend in general on the stationary distribution of an induced Markov chain that is derived from the underlying queueing system. We illustrate our results with examples and simulation experiments. We also demonstrate that the stability of the LQF depends on the tie-breaking rule used and that it can be unstable even under arbitrary low loads.     

Synthesis and immobilization of micro-scale drug particles in cellulosic films

The anti-solvent synthesis of micron-scale particles, their stabilization, and subsequent self-assembly into polymer films suitable for drug delivery is presented. The colloidal particles were stabilized using low molecular weight hydroxypropyl methylcellulose (HPMC), while drug encapsulation was carried out with high molecular weight HPMC and polyvinylpyrrolidone (PVP). Griseofulvin (GF) was used as the model drug compound, and the polymer films were evaluated in terms of their surface morphology, mechanical properties and in vitro drug release. In general, the release rates were best described by first-order and Hixson-Crowell kinetic models, and in a typical film containing 57% HPMC, 100% of GF was released within 50 min.