Author ORCID Identifier
0000-0002-7263-7903
Document Type
Dissertation
Date of Award
12-31-2025
Degree Name
Doctor of Philosophy in Mechanical Engineering - (Ph.D.)
Department
Mechanical and Industrial Engineering
First Advisor
Fatemeh Ahmadpoor
Second Advisor
Dibakar Datta
Third Advisor
Farid Alisafaei
Fourth Advisor
Samaneh Farokhirad
Fifth Advisor
Camelia Prodan
Abstract
Nanoscale structures are inherently dynamic due to persistent thermal fluctuations. Even in solid materials, these random deformations, driven by ambient thermal energy, can become significant when their characteristic scale approaches that of the structure itself and strongly influence their overall mechanical behavior. Examples of such structures include crystalline membranes, appearing in diverse forms such as nanotubes, nanoribbons, and kirigami/origami structures. Similarly, biological nanostructures, including lipid membranes, microtubules, actin filaments, and DNA, exhibit extreme flexibility and responsiveness due to their low bending rigidity. Numerous physiological processes are intrinsically linked to these thermal fluctuations, including exocytosis and endocytosis, membrane fusion, pore formation, cell adhesion, binding and unbinding transitions, self-assembly, vesicle size distributions, red blood cell membrane configurations, and the cytoskeletal or actin mediated mechanics of membranes. These biophysical processes arise from the balance of attractive and repulsive forces between biological structures. Van der Waals forces, which act even between rigid membranes, provide long range attraction, scaling as 1/d3 at short and 1/d6 at larger distances. In flexible membranes, an additional repulsive force known as the entropic force emerges from thermal fluctuations. Entropic force, a long standing topic in biophysics and biomechanics, has been studied for over four decades. Similar to an ideal gas, fluctuating surfaces generate entropic pressure due to thermal fluctuations. Previous studies were mostly limited to fluid, planar, tension free membranes and assumed passive behavior, while real biological membranes often experience activity that enhances their fluctuations. This dissertation offers a comprehensive investigation, addressing key challenges such as surface tension, confinement, membrane size, curvature, nonlinear elasticity, and active forces. Using statistical mechanics, continuum mechanics and computational models for both fluid and solid membranes, we explore how biological factors influence membrane fluctuations and entropic repulsion, with implications for membrane interactions with external objects, nanostructures, viral capsids, and other cells.
Recommended Citation
Hassan, Rubayet, "Entropic forces near fluctuating surfaces" (2025). Dissertations. 1864.
https://digitalcommons.njit.edu/dissertations/1864
Included in
Biochemistry, Biophysics, and Structural Biology Commons, Mechanical Engineering Commons, Mechanics of Materials Commons
