@article{wu_gong_szleifer_vlcek_subr_genzer_2007, title={Behavior of surface-anchored poly(acrylic acid) brushes with grafting density gradients on solid substrates: 1. Experiment}, volume={40}, ISSN={["1520-5835"]}, DOI={10.1021/ma0710176}, abstractNote={We describe experiments pertaining to the formation of surface-anchored poly(acrylic acid) (PAA) brushes with a gradual variation of the PAA grafting densities on flat surfaces and provide detailed analysis of their properties. The PAA brush gradients are generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by the “grafting from” polymerization of tert-butyl acrylate (tBA) from these substrate-bound initiator centers, and finally converting the PtBA into PAA. We use spectroscopic ellipsometry to measure the wet thickness of the grafted PAA chains in aqueous solutions at three different pH values (4, 5.8, and 10) and a series of ionic strengths (IS). Our measurements reveal that at low grafting densities, σ, the wet thickness of the PAA brush (H) remains relatively constant, the polymers are in the mushroom regime. Beyond a certain value of σ, the macromolecules enter the brush regime, where H increases with increasing σ. For a given σ, H exhibits a nonmonotonic behavior as a function of the IS. At large IS, the H is small because the charges along PAA are completely screened by the excess of the external salt. As IS decreases, the PAA enters the so-called salt brush (SB) regime, where H increases. At a certain value of IS, H reaches a maximum and then decreases again. The latter is a typical brush behavior in so-called osmotic brush (OB) regime. We provide detailed discussion of the behavior of the grafted PAA chains in the SB and OB regimes.}, number={24}, journal={MACROMOLECULES}, author={Wu, Tao and Gong, Peng and Szleifer, Igal and Vlcek, Petr and Subr, Vladimir and Genzer, Jan}, year={2007}, month={Nov}, pages={8756–8764} }
 @article{wan_wu_chastain_roberts_kuznetsov_ro_2005, title={Forced convective cooling via acoustic streaming in a narrow channel established by a vibrating piezoelectric bimorph}, volume={74}, ISSN={["1573-1987"]}, DOI={10.1007/s10494-005-4132-4}, abstractNote={Forced convection in a narrow channel is investigated both numerically and experimentally. The flow field is established through the mechanism of acoustic streaming. This is accomplished by high frequency vibration of one of the channel walls, which is composed of a piezoelectric bimorph. In the numerical computations, the Navier-Stokes equations are decomposed into the acoustic equations and the streaming equations by the perturbation method. The acoustic field is first numerically obtained, which provides the driving force for the streaming field. The streaming field and the associated temperature field are then obtained numerically. Heat losses from a heat source are measured to determine the efficiency of this as a cooling method. The air-flow patterns in the channel between the heat source and the bimorph actuator are visualized using the particle tracking velocimetry. The visualization clearly shows that vortical streaming (acoustic streaming) can be induced by bimorph vibration, which enhances heat transfer between the heat source and the surrounding air. The temperature decreases obtained computationally and experimentally are in good agreement.}, number={2}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Wan, Q and Wu, T and Chastain, J and Roberts, WL and Kuznetsov, AV and Ro, PI}, year={2005}, month={Mar}, pages={195–206} }
 @article{wu_ro_2005, title={Heat transfer performance of a cooling system using vibrating piezoelectric beams}, volume={15}, ISSN={["1361-6439"]}, DOI={10.1088/0960-1317/15/1/030}, abstractNote={A prototype of a miniature cooling system for microelectronics using vibrating piezoelectric beam was proposed and constructed. The flow patterns and cooling effects of the system were investigated experimentally. The vibration characteristics of the piezoelectric beam were simulated by the finite element method. Cooling effects were measured in terms of the temperature drop of the heat source above the vibrating beam. The electric field applied on the piezoelectric beam and the gap between heat source and actuator were adjusted to find the best cooling result. A temperature drop of 25.9 °C from 92 °C can be observed for the heat source when operating at a certain condition, which indicates the feasibility of using the present miniature cooling system in small devices. The heat transfer performance of the cooling system was analyzed as well. The results show that the enhancement of heat transfer between the heat source and the beam can be up to 210% with the acoustic streaming generated by the beam vibration in the present experimental studies. The enhanced heat transfer can be attributed to the generation of vortical streaming by the vibrating beam which was also captured in the flow visualization experiments.}, number={1}, journal={JOURNAL OF MICROMECHANICS AND MICROENGINEERING}, author={Wu, T and Ro, PI}, year={2005}, month={Jan}, pages={213–220} }
 @article{wu_ro_2004, title={Dynamic peak amplitude analysis and bonding layer eiiects of piezoelectric bimorph cantilevers}, volume={13}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/13/1/024}, abstractNote={An analytical prediction of dynamic performance for piezoelectric bimorph structures was investigated. A damping ratio was assumed and employed to determine dynamic peak amplitudes at resonances by finding the frequency with of the peak amplitude. Finite element simulations were used to validate the proposed improvement strategy. Results show that the peak amplitude determination method is good enough to predict the dynamic performance of piezoelectric bimorphs. The effects of bonding layers were also analysed by both static and dynamic methods. The bonding influence can be minimized by selecting appropriate bonding materials and dimensions of structures.}, number={1}, journal={SMART MATERIALS AND STRUCTURES}, author={Wu, T and Ro, PI}, year={2004}, month={Feb}, pages={203–210} }
 @article{wu_tomlinson_efimenko_genzer_2003, title={A combinatorial approach to surface anchored polymers}, volume={38}, ISSN={["1573-4803"]}, DOI={10.1023/A:1027373216389}, number={22}, journal={JOURNAL OF MATERIALS SCIENCE}, author={Wu, T and Tomlinson, M and Efimenko, K and Genzer, J}, year={2003}, month={Nov}, pages={4471–4477} }
 @article{wu_efimenko_vlcek_subr_genzer_2003, title={Formation and properties of anchored polymers with a gradual variation of grafting densities on flat substrates}, volume={36}, ISSN={["1520-5835"]}, DOI={10.1021/ma0257189}, abstractNote={We show that assemblies comprising anchored polymers with a gradual variation of grafting densities on solid substrates can be generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by polymerization from the substrate-bound initiator centers (“grafting from”). We apply this technique to prepare grafting density gradients of poly(acrylamide) (PAAm) on flat silica substrates. We demonstrate that using the grafting density gradient geometry, the mushroom-to-brush transition can be accessed on a single sample. This transition is detected by monitoring the dependence of the thickness of the grafted PAAm in a good solvent using variable angle spectroscopic ellipsometry. Wettability experiments performed on the gradient PAAm substrate provide complementary information about the nature of the mushroom-to-brush transition.}, number={7}, journal={MACROMOLECULES}, author={Wu, T and Efimenko, K and Vlcek, P and Subr, V and Genzer, J}, year={2003}, month={Apr}, pages={2448–2453} }
 @article{wu_efimenko_genzer_2002, title={Combinatorial study of the mushroom-to-brush crossover in surface anchored polyacrylamide}, volume={124}, ISSN={["0002-7863"]}, DOI={10.1021/ja027412n}, abstractNote={We present a method for fabricating anchored polymers with a gradual variation of grafting densities on solid substrates. The technique for generating such structures comprises (i) formation of a molecular gradient of polymerization initiator on the solid substrate and (ii) polymerization from the substrate-bound initiator centers ("grafting from"). We measure the mushroom-to-brush transition in grafted polyacrylamides and show that the mushroom and brush behavior can be described using existing scaling theories.}, number={32}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Wu, T and Efimenko, K and Genzer, J}, year={2002}, month={Aug}, pages={9394–9395} }