The apparent contact angle on a rough surface is usually modeled by either Cassie's or Wenzel's theory. We show, on the basis of experimental evidence, that there can be two contact angles on the same rough surface, depending on how a drop is formed. A transition can occur between the different states by an external disturbance. This paper compares the theoretical prediction with matching experiments. This leads to the establishment of a design criterion for a robust hydrophobic rough surface on which the apparent contact angle will not change as a result of an external disturbance.

No abstract

Ambipolar polymer semiconductors are highly suited for use in flexible, printable, and large-area electronics as they exhibit both n-type (electron-transporting) and p-type (hole-transporting) operations within a single layer. This allows for cost-effective fabrication of complementary circuits with high noise immunity and operational stability. Currently, the performance of ambipolar polymer semiconductors lags behind that of their unipolar counterparts. Here, we report on the side-chain engineering of conjugated, alternating electron donor-acceptor (D-A) polymers using diketopyrrolopyrrole-selenophene copolymers with hybrid siloxane-solubilizing groups (PTDPPSe-Si) to enhance ambipolar performance. The alkyl spacer length of the hybrid side chains was systematically tuned to boost ambipolar performance. The optimized three-dimensional (3-D) charge transport of PTDPPSe-Si with pentyl spacers yielded unprecedentedly high hole and electron mobilities of 8.84 and 4.34 cm(2) V(-1) s(-1), respectively. These results provide guidelines for the molecular design of semiconducting polymers with hybrid side chains.

There is a fast-growing demand for polymer-based ambipolar thin-film transistors (TFTs), in which both n-type and p-type transistor operations are realized in a single layer, while maintaining simplicity in processing. Research progress toward this end is essentially fueled by molecular engineering of the conjugated backbones of the polymers and the development of process architectures for device fabrication, which has recently led to hole and electron mobilities of more than 1.0 cm(2) V(-1) s(-1). However, ambipolar polymers with even higher performance are still required. By taking into account both the conjugated backbone and side chains of the polymer component, we have developed a dithienyl-diketopyrrolopyrrole (TDPP) and selenophene containing polymer with hybrid siloxane-solubilizing groups (PTDPPSe-Si). A synergistic combination of rational polymer backbone design, side-chain dynamics, and solution processing affords an enormous boost in ambipolar TFT performance, resulting in unprecedentedly high hole and electron mobilities of 3.97 and 2.20 cm(2) V(-1) s(-1), respectively.

No abstract

No abstract

This paper summarizes the newly developed immersed finite element method (IFEM) and its applications to the modeling of biological systems. This work was inspired by the pioneering work of Professor T.J.R. Hughes in solving fluid-structure interaction problems. In IFEM, a Lagrangian solid mesh moves on top of a background Eulerian fluid mesh which spans the entire computational domain. Hence, mesh generation is greatly simplified. Moreover, both fluid and solid domains are modeled with the finite element method and the continuity between the fluid and solid subdomains is enforced via the interpolation of the velocities and the distribution of the forces with the reproducing Kernel particle method (RKPM) delta function. The proposed method is used to study the fluid-structure interaction problems encountered in human cardiovascular systems. Currently, the heart modeling is being constructed and the deployment process of an angioplasty stent has been simulated. Some preliminary results on monocyte and platelet deposition are presented. Blood rheology, in particular, the shear-rate dependent de-aggregation of red blood cell (RBC) clusters and the transport of deformable cells, are modeled. Furthermore, IFEM is combined with electrokinetics to study the mechanisms of nano/bio filament assembly for the understanding of cell motility.

Purpose: Design and optimization of medical imaging systems benefit from accurate theoretical modeling that identifies the physical factors governing image quality, particularly in the early stages of system development. This work extends Fourier metrics of imaging performance and detectability index ͑dЈ͒ to tomosynthesis and cone-beam CT ͑CBCT͒ and investigates the extent to which dЈ is a valid descriptor of task-based imaging performance as assessed by human observers. Methods:The detectability index for tasks presented in 2D slices ͑d slice Ј ͒ was derived from 3D cascaded systems analysis of tomosynthesis and CBCT. Anatomical background noise measured in a physical phantom presenting power-law spectral density was incorporated in the "generalized" noise-equivalent quanta. Theoretical calculations of d slice Ј were performed as a function of total angular extent ͑ tot ͒ of source-detector orbit ranging 10°-360°under two acquisition schemes: ͑i͒ Constant angular separation between projections ͑constant-⌬͒, giving variable number of projections ͑N proj ͒ and dose vs tot and ͑ii͒ constant number of projections ͑constant-N proj ͒, giving constant dose ͑but variable angular sampling͒ with tot . Five simple observer models were investigated: Prewhitening ͑PW͒, prewhitening with eye filter and internal noise ͑PWEi͒, nonprewhitening ͑NPW͒, nonprewhitening with eye filter ͑NPWE͒, and nonprewhitening with eye filter and internal noise ͑NPWEi͒. Human observer performance was measured in 9AFC tests for five simple imaging tasks presented within uniform and power-law clutter backgrounds. Measurements ͑from 9AFC tests͒ and theoretical calculations ͑from cascaded systems analysis of d slice Ј ͒ were compared in terms of area under the ROC curve ͑A z ͒ Results: Reasonable correspondence between theoretical calculations and human observer performance was achieved for all imaging tasks over the broad range of experimental conditions and acquisition schemes. The PW and PWEi observer models tended to overestimate detectability, while the various NPW models predicted observer performance fairly well, with NPWEi giving the best overall agreement. Detectability was shown to increase with tot due to the reduction of out-of-plane clutter, reaching a plateau after a particular tot that depended on the imaging task. Depending on the acquisition scheme, however ͑i.e., constant-N proj or ⌬͒, detectability was seen in some cases to decline at higher tot due to tradeoffs among quantum noise, background clutter, and view sampling. Conclusions: Generalized detectability index derived from a 3D cascaded systems model shows reasonable correspondence with human observer performance over a fairly broad range of imaging tasks and conditions, although discrepancies were observed in cases relating to orbits intermediate to 180°and 360°. The basic correspondence of theoretical and measured performance supports the 1754 1754 Med. Phys. 38 "4…,