11/21/2022
By Murat Inalpolat

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a doctoral dissertation defense by Darshil Shah on “Highly Filled Photopolymers for Metamaterial Applications and its Light Scattering Behavior.”

Candidate Name: Darshil Shah
Defense Date: Monday, Dec. 5, 2022
Time: 2 to 3 p.m. EDT
Location: Dandeneau 220 and virtual via MS Teams

This will be a virtual defense via MS Teams and in person at DAN 220. Those interested in attending should contact Darshil_Shah@student.uml.edu and the committee chair Christopher_Hansen@uml.edu at least 24 hours prior to the defense to request access to the meeting.

Committee:

  • Advisor Christopher Hansen, Ph.D., Professor, Chair, Department of Mechanical Engineering, University of Massachusetts Lowell
  • Alireza Amirkhizi, Ph.D., Associate Professor, Department of Mechanical Engineering, University of Massachusetts Lowell
  • Amy Peterson, Ph.D., Associate Professor, Department of Plastics Engineering, University of Massachusetts Lowell
  • Robert. Jensen, Ph.D., Team Lead, Materials Data Science, DEVCOM Army Research Laboratory (ARL)

Brief Abstract:

Additive Manufacturing (AM), popularly known as 3D printing, has seen a meteoric rise in both hardware and software technology development and materials proliferation in the last decade, enabling applications not previously feasible. Mechanical metamaterials is an application that has seen growing interest due to the ability to create complex geometries with fine features. Mechanical metamaterials are specifically tailored unit cell array structures with the potential to control wave propagation, as these structures exhibit dynamic behavior not normally observed in homogeneous and natural materials. The performance of a metamaterial depends both on its geometric structure and on its intrinsic material properties, such as density and modulus. Digital Light Processing (DLP)-based 3D printing enables the printing of mechanical metamaterial structures with relatively high resolution. While enabling metamaterial fabrication, photopolymers used in DLP-based 3D printing are limited in their intrinsic properties, thereby limiting the design space of metamaterials. The addition of fillers, especially at higher volume fractions, allows the tailoring of photopolymer material properties. Lowering the density, while maintaining or even increasing the modulus, improves the feasibility for future commercial applications such as acoustic cloaking, sound insulation, and blast wave attenuation. Commercially, however, no low-density resins exist for DLP-based 3D printing. Further improvements in metamaterial performance could be achieved by printing the unit cell array with multiple materials with contrasting densities.

The main objectives of this dissertation are to (1) prepare highly filled syntactic foam material systems using hollow glass microspheres for DLP-based 3D printing, (2) characterize the light scattering within these filled systems via modeling to predict cured dimensions, and (3) to develop a methodology to fabricate multi-material metamaterial specimens with divergent properties on a single layer.

The first part of this dissertation aims to understand the effect of dispersants on the suspension stability of highly filled photopolymer resins over print times and filler homogeneity in the fabricated structure. The material is further characterized to optimize its processing parameters and understand the overall print quality for mechanical metamaterial applications.

The addition of hollow-glass microspheres to a photopolymer resin induces light scattering within a three-phase system that is not captured by existing two-phase models. The second topic studied for the first time in this dissertation is the modeling of light scattering using a first-principles approach for systems with three optically active phases, i.e., the surrounding photopolymer resin, the glass shell, and the air core from the hollow-glass filler. Mie scattering theory is utilized to characterize the scattering property of a single hollow-glass sphere. This scattering distribution is input into the Monte Carlo method to scale up the scattering behavior at different volume fractions of hollow-glass microspheres. The model results are compared to fabricated prints.

Finally, the third aim of this dissertation is to study the multi-material DLP-based printing of resins with dramatically different processing parameters. An open-source, Autodesk Ember DLP printer is modified to investigate a multi-vat approach for multi-material printing of two resins -- a commercially available, high-density, PR48 resin, and a 50 % filled, low-density resin with viscosities differing by 3 orders of magnitude. A mechanical metamaterial is fabricated and characterized for its overall frequency response as a demonstration.

All interested students and faculty members are invited to attend the online defense via remote access.