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SUMMARY:PhD Defense\, MD Anisur Rahman\, “ENGINEERING ROUGH COLLOIDS TO 
 CONTROL INTERFACIAL PINNING\, CAPILLARY INTERACTIONS\, AND MONOLAYER BEHAV
 IOR"
DTSTART;VALUE=DATE-TIME:20261030T160000Z
DTEND;VALUE=DATE-TIME:20261030T173000Z
UID:152418483494
DESCRIPTION:Chair:  Peter BeltramoAbstractColloidal particles trapped at 
 fluid interfaces have numerous applications\, ranging from emulsion stabil
 ity to fabricating two-dimensional (2D) metamaterials. Their microstructur
 al organization dictates resultant material properties\, such as increased
  yield stress of kinetically arrested aggregates or the emergence of optic
 al metasurface effects from ordered assemblies. These microstructures may 
 be engineered by manipulating particle surface topography or shape anisotr
 opy to control how individual particles pin at the interface and subsequen
 t interparticle interactions. This thesis investigates how particle surfac
 e roughness affects the interfacial pinning and interactions of shape-isot
 ropic spheres and shape-anisotropic ellipsoids\, and examines how these ef
 fects govern collective monolayer behavior\, including interfacial assembl
 y and mechanics.  By rationally designing spherical and ellipsoidal polym
 er colloids with controlled roughness and topography (convex or concave)\,
  we demonstrate that the effect of roughness on interfacial pinning and de
 formation depends on particle shape. For spheres\, surface roughness enhan
 ces quadrupolar interfacial deformation and thereby capillary attraction\,
  as theoretically predicted. Surprisingly\, for ellipsoids\, roughness sup
 presses quadrupolar deformation and thus weakens capillary attraction betw
 een rough ellipsoids relative to their smooth analogues.  To interrogate 
 how these single-particle and pairwise behaviors influence multiparticle a
 ssembly\, we slowly compacted a monolayer of particles at an air-water int
 erface using a Langmuir trough and simultaneously measured the monolayer s
 urface pressure and optically imaged the resultant microstructures. With i
 ncreasing roughness\, surface pressure increases for spheres and decreases
  for ellipsoids\, consistent with our previously measured trends in capill
 ary interaction between particles. However\, optical microscopy reveals th
 at the specific topography\, in addition to shape and roughness\, dictates
  monolayer microstructure. Notably\, while convex and concave ellipsoids b
 oth show attenuated capillary attraction\, only concave rough ellipsoids f
 orm orientationally ordered monolayers at close packing\, as opposed to th
 e disordered multiparticle aggregates formed by smooth and convex ellipsoi
 ds. Surface topography therefore influences particle organization beyond s
 imply reducing capillarity.  Finally\, we experimentally determine the as
 pect ratio (AR)-area fraction assembly phase diagram of concave rough elli
 psoids. As area fraction increases\, monolayers undergo a disorder-order t
 ransition from isotropic to plastic crystal phases at low AR and an isotro
 pic to nematic transition at high AR\, agreeing qualitatively with theoret
 ical predictions for hard ellipses. These assemblies can be successfully t
 ransferred onto solid substrates via Langmuir-Blodgett deposition\, demons
 trating the potential of using rough ellipsoids to create 2D ordered aniso
 tropic patterns through interfacial templating. Ultimately\, this thesis c
 omprehensively establishes using rough particles to engineer fluid interfa
 ces by combining particle synthesis\, quantitative individual particle pin
 ning and interaction measurements\, and evaluation of monolayer mechanics 
 and assembly. These findings provide a route to complex\, ordered 2D aniso
 tropic microstructures through particle surface engineering and assembly a
 t fluid interfaces and lay the groundwork for creating scalable anisotropi
 c metasurfaces.
LOCATION:Guness Student Center inside Marcus Hall & via Zoom
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