The frequent outbreaks of foodborne and waterborne diseases have become a serious public health issue, which is related to the presence of microbial biofilms on surfaces of materials. Current research activities aiming at reducing or eliminating the biofilm formation could be mainly divided into two major approaches: an active approach of “attacking”, by killing the microbes once they are attached to the material surface; and a passive approach of “defending”, creating resistance to microbial attachment or releasing the attached one. In this dissertation, a dual-functional structure was initially proposed: rechargeable biocidal moieties immobilized at the inner layer, and antifouling moieties serving as a protective layer located on the outside of the surface. With a such unique surface structure, once microorganisms approach the designed surfaces, (i) the antifouling moieties could reduce the non-specific adsorption of the microorganisms and make them difficult to adhere; (ii) any live bacteria attached on the surface could be immediately killed by the incorporated biocidal moieties; (iii) any resulting cell debris and other biofoulants could be easily washed off with mild hydrodynamic force applied; furthermore, (iv) the consumed biocidal activity could be regenerated by either chlorination or photoirradiation. Overall, I designed and successfully developed three types of rechargeable biocidal and antifouling functional materials, which exhibited great potentials in food packaging, water filtration, and medical application areas.Specifically, in this dissertation, chapter 1 briefly introduces processes and mechanisms of microbial biofilm formation, two major strategies responding to avoid and reduce the formation of biofilm, and typical chemicals with rechargeable biocidal or antifouling functions applied in recent years. In chapter 2, a new antibiofilm concept combining both rechargeable antimicrobial and antifouling functions was proposed. An N-halamine precursor, N,N-diallylmelamine (DAM), and a zwitterionic monomer, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), were covalently bonded onto poly(vinyl alcohol-co-ethylene) (PVA-co-PE) and generated a double-layer structure. The outside surface embedding with resisting and releasing functions and the inner layer with regenerable biocidal activities work constructively to limit the biofilm development. In chapter 3, a new synthesized zwitterionic polymer, sulfonated polyethyleneimine (PEI-S), was chemically bonded with an N-halamine grafted PVA-co-PE nanofibrous membrane. The obtained PEI-S@BNF nanofibrous membrane with the super-high specific surface area did effectively magnify the interfacial effect. Two engineering models were constructed to explain the resisting and releasing mechanisms via typical isothermal adsorption and release experiments. Additionally, the final product, PEI-S@BNF nanofibrous membrane, showed great potential to be applied as a water filtration material. In chapter 4, instead of N-halamine, a photo-induced biocide, benzophenonetetracarboxylic dianhydride (BPTCD) was incorporated onto the PVA-co-PE nanofibrous membranes, providing a photo-driven regenerable biocidal function. After combined with antifouling moieties, the obtained SBMA@EVOH also exhibited a desirable antibiofilm effect, which proved the universality of such proposed bifunctional antibiofilm strategy. In chapter 5, a N-halamine-based chlorine rechargeable antimicrobial composite fabric (HCF) was fabricated via an industrial scalable dip-coating method. An imide/amide halamine grafted cotton fabric (DMH-g-cotton) served as a reinforcement component and a reservoir of active chlorine. And an amine halamine polymer (PVA-co-PE-g-DAM) as a coating component could reduce the loss of active chlorine, realizing long-term killing performance. With such a unique combination, the designed HCF could be considered as a potential active food-contact material that could effectively reduce microbial cross-contamination, prolong the food lifetime, decrease food spoilage, and ensure food safety. The final chapter describes all achievements related to the proposed multifunctional double-layer surface structure, which could significantly reduce the biofouling effect via resisting, killing, and releasing pathways, and a practical demonstration of biocidal functional composite fabric applied as a food-contact material.

Extensive deployment of power electronic loads in naval ship power systems indicate full ship electrification is inevitable. Next generation warships require high power density weapons drawing pulse power from a dc power grid. A particularly concerning issue is that these pulse loads draw large currents in short periods of time and are similar in behavior to a fault; and therefore may be indiscernible from a fault. This dissertation introduces novel machine/deep learning based algorithms, including long short-term memory recurrent neural network based autoencoders and data-driven clustering based machine learning approaches to detect dc faults and monitor load conditions applied to naval pulse loads. Two feature extraction methods are also implemented including the short-time Fourier transform and stationary wavelet transform. The novel load monitoring solution presented herein can be applied to any load profile that exhibits repetitive transients during normal operation. The frequency-domain features of the load current are extracted for the network training to set the network weights and biases. Once the network training is completed, the machine/deep learning approach will predict both signal classification and fault identification. Finally, the method is demonstrated in a low power laboratory system meant to mimic naval shipboard power systems.

The Mozambique Channel is a conduit of trade wind-driven, warm and salty throughflow that is a key component of the Agulhas Current and Agulhas leakage. Agulhas leakage is a flux of warm and salty water from the tropical Indian to the Atlantic Ocean and is thought to strengthen the Atlantic meridional overturning circulation (AMOC). Previous studies from the Agulhas region suggest that enhanced Agulhas leakage, driven by shift and intensification of the southern hemisphere westerlies, played an important role in accelerating glacial terminations. The southeastern monsoon response to freshwater-induced climate instabilities that punctuated the last deglaciation and its impact on the Mozambique Channel through flow and, by extension, on the Agulhas leakage is not well understood. Here we present a high-resolution hydroclimate and mixed layer temperature reconstruction using three sediment cores collected from the runoff-influenced eastern Mozambique Channel, off northwestern Madagascar, in the core region of the southern hemisphere monsoon domain. Local δ18O seawater time-series, obtained by removing the calcification temperature and ice volume imprints on δ18O of Globigerinoides ruber tests, is used as a proxy of river runoff and, by extension, precipitation changes over northwestern Madagascar. Superimposed on orbital scale climate trend that indicates a humid glacial, the proxy record reveals precipitation increases centered at 11-13 thousand years before present (kyr BP), 14-18 kyr BP, and 23-25 kyr BP. Considering age model uncertainty, this is the first strong and consistent evidence for southern African monsoon strengthening in response to freshwater forcing during Younger Dryas (YD), Heinrich Event 1 (H1), and Heinrich Event 2 (H2). Furthermore, our study shows a reversal of mixed layer temperature gradient between the west and east Mozambique Channel during H1 and YD. Consistent with the results of climate simulation, we suggest that the gradient reversal indicates a weakening of the trade wind-driven South Equatorial Current and Mozambique Channel throughflow that weakens the Agulhas leakage. In conclusion, the results of this study indicate that millennial scale weakening of Mozambique Channel through flow is strongly linked to the changes in the southern hemisphere monsoon wind fields over the Tropical Indian Ocean. As shown in this study, the latter is sensitive to northern high latitude freshwater forcing, potential creating a positive feedback that contributes to the weakening of the AMOC by reducing the amount of heat and salt leakage into the Atlantic.

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