Mr. Navid Namdari Profile

Navid Namdari

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Proceedings Article | 24 April 2020 Presentation

Facile fabrication of nano-featured superhydrophobic surfaces by damage induced surface texturing of nano-composites (Conference Presentation)

Navid Namdari, Ravi Sadanala, Hossein Sojoudi, Reza Rizvi

Proceedings Volume 11378, 113780D (2020) https://doi.org/10.1117/12.2558987

KEYWORDS: Nanolithography, Nanocomposites, Carbon, Composites, Polymers, Nanofibers, Resistance, Matrices, Californium

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Damage and mechanical failure are detrimental for materials in engineering applications. However, recent studies have shown that properly designed debonding and pullout phenomena in composite fractures can be implemented to introduce added functionalities by damage induced surface texturing (DIST). In the DIST approach, randomly oriented or aligned fibers are incorporated in a polymeric matrix, followed by transversal shearing of the surface. As a result, the shear-fractured surface will have protruded fibers on it because of debonding followed by subsequent pullout. The protruded fibers play a key role in imparting new functionalities to the cut surface. One of the drawbacks of DIST is that the introduced surface functionalities (i.e. hydrophobicity) are affected by mechanical weathering since the commercial carbon fibers used lack abrasion and flexural fatigue resistance. In order to overcome this issue, carbon nanofibers (CNFs) can be used resulting in enhancing the surface wettability of generated surfaces from hydrophobic to the superhydrophobic regime. We have shown that by using 36 wt.% chemical vapor grown hollow carbon nanofibers in styrene-ethylene/butylene-styrene (SEBS) matrices the water sessile-drop contact angle of pristine cut SEBS samples increases by 41% from 106.1˚ ± 2.9˚ to 151.7˚ ± 1.7˚. Although the CNFs are small in diameter size (100 nm) with respect to CFs having two orders of magnitude larger diameters, they provide a high specific surface area of up to ~ 50 m2/g while the latter has a specific area of ~ 1 m2/g. The higher surface area generated by protruded CNFs on the surface can introduce air pockets between the surface and water droplets necessary for surface supperwettability. DIST is a simple, economical, and scalable. In addition, there is no need to use costly and time-consuming post-processing stages to introduce anisotropic properties into the material.

Proceedings Article | 22 April 2020 Presentation + Paper

Visual stress sensing using dynamic porosity in shape memory thermoplastic elastomers

Navid Namdari, Bilal Nizar Abdul Halim, Sheikh Rasel, Hossein Sojoudi, Reza Rizvi

Proceedings Volume 11377, 113770Q (2020) https://doi.org/10.1117/12.2558969

KEYWORDS: Polymers, Solids, Visualization, Manufacturing, Shape memory polymers, Optical properties, Optical sensing

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Shape memory polymers (SMP) have been extensively implemented for applications ranging from biomedical devices and soft robotics to deployable structures. However, these materials mostly rely on heat for shape recovery and programming. Here, we introduce a new class of smart SMPs with pressure-responsive characteristics based on dynamic porosity–an ability to configure a pore size within a solid– that allows for optical monitoring of the stimuli. Introducing pores in a structure reduces the relative density of the material and introduces interfaces resulting in a reduction in optical transparency of the bulk through light scattering. We show that utilizing the dynamic porosity, a macroporous film transitions instantly from an optically opaque (25% transmittance) to a transparent (96% transmittance) state by applying an out-of-plane contact pressure of 3.8 MPa at room temperature. The new SMP also presents an anomalous ‘cold’ shape recovery of the pressure-activated films through repeated in-plane stretching that renucleates the pores and transitions the material back to the initially opaque state. The presented concept ushers a new class of responsive materials that interlace optical sensitivity features with micro- and nanoscale material deformations, establishing new research opportunities in the field of SMPs.

Proceedings Article | 29 March 2019 Paper

Porous materials with stress and temperature activated porosity

Navid Namdari, Bilal Nizar Abdul Halim, Sheikh Rasel, Md Emran Hossain Bhuiyan, Raghavendra Alwala, Reza Rizvi

Proceedings Volume 10968, 109680X (2019) https://doi.org/10.1117/12.2515380

KEYWORDS: Foam, Carbon monoxide, Scanning electron microscopy, Polymers, Opacity, Transparency, Transmittance, Solid state electronics

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The fabrication of micro-structured films with dynamic porosity is useful in various applications. This article presents fabrication of microcellular films exhibiting dynamic porosity upon applying an external pressure stimulus. Interestingly, we show that the dynamic porosity is in conjunction with an opaque to transparent transition (OTT) that imparts the material with a unique optomechanical behavior. The OTT experienced foams will have a transparency almost equal of the as-cast films. The foaming process used for this study is solid-state foming technique with CO₂ gas as physical blowing agent. A styrene-ethylene-butylene-styrene (SEBS) triblock copolymer was used as the foaming material. Polystyrene (PS) blocks in SEBS play a key role in this process since CO₂ reduces the T_g of PS and enable ethylenebutylene (EB) parts to swell during saturating stage while upon depressurization their T_g increases again that prevents the forming pores from collapse. A custom-made in-situ optomechanical setup was used to characterize the optical and mechanical behavior of the produced foams. Optomechanical tests at different strain rates and loads revealed that the films undergo a gradual OTT behavior indicating their ability to be used as optical pressure-sensitive films. Quenching temperature is of great importance in this process since the foams show various OTT behavior once the quenching temperature changes. Moreover, here we show after pressing the foams, the transparent films can be re-foamed by the same process for many cycles.

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