Debojit Chanda | Searcharxiv

arXiv2024

Optical Micromanipulation of Soft Materials: Applications in Devices and Technologies

Since its invention by Arthur Ashkin and colleagues at Bell Labs in the 1970s, optical micromanipulation, also known as optical tweezers or laser tweezers, has evolved remarkably to become one of the most convenient and versatile tools for studying soft materials, including biological systems. Arthur Ashkin received the Nobel Prize in Physics in 2018 for enabling these extraordinary scientific advancements. Essentially, a focused laser beam is used to apply and measure minuscule forces from a few piconewtons to femtonewtons by utilizing light-matter interaction at mesoscopic length scales. Combined with advanced microscopy and position-sensing techniques, optical micromanipulations enable us to investigate diverse aspects of functional soft materials. These include studying mechanical responses through force-elongation measurements, examining the structural properties of complex fluids employing microrheology, analyzing chemical compositions using spectroscopy, and sorting cells through single-cell analysis. Furthermore, it is utilized in various soft-matter-based devices, such as laser scissors and optical motors in microfluidic channels. This chapter presents an overview of optical micromanipulation techniques by describing fundamental theories and explaining the design considerations of conventional single-trap and dual-trap setups as well as recent improvisations. We further discuss their capabilities and applications in probing exotic soft-matter systems and in developing widely utilized devices and technologies based on functional soft materials.

cond-mat.soft↗

arXiv2024

Entropic crystallization of Brownian squares through pathways governed by orientational dynamics

In dense systems of hard-interacting colloidal particles having anisotropic shapes, crystallization pathways represent an interesting frontier. The translational and rotational dynamics of such particles become coupled at higher densities, resulting in complex kinetics of their configurational ordering. To elucidate this, we have studied a two-dimensional entropic system of osmotically compressed corner-rounded Brownian square platelets. By analyzing the translational and orientational dynamics of the particles and their respective contributions toward minimizing the free energy, we show that the range of accessible orientational states of the particles principally governs the pathways of structural evolution, as the orientational entropy dictates the minimization of the free energy and, hence, the resulting optimal equilibrium ordering. When the particles have access to a wider range of orientational states, the larger rotational component of configurational entropy minimizes the total free energy, leading to hexagonal ordering. At higher osmotic pressures, the long collective translational fluctuations of the side-aligned particles with restricted rotational fluctuations maximize the entropy with a greater contribution from the translational component, thereby inducing a free energetically favored rhombic crystalline structure. We further show that density influences the crystallization pathways indirectly by setting an upper bound on the range of accessible orientational states. Complementary Brownian dynamics simulations and free-energy calculations further corroborate our findings, and their generalizability is demonstrated using a system of triangular particles. Thus, orientational dynamics is predicted to play a crucial role in governing the pathways for entropic ordering of various anisotropic shapes.

cond-mat.soft↗