Formation of Asymmetric Colloidal Multilayers via Subduction of Laterally Segregated Domains at the Air/Water Interface

We present an interfacial assembly strategy for constructing asymmetric multilayered colloidal films through lateral compression of laterally segregated particle microdomains at the air–water interface. These microdomains—composed of polystyrene (PS) and silica (SiO₂) particles—serve as lateral templates that direct vertical rearrangement during monolayer collapse. Utilizing hydrophilic PS and SiO₂ particles with distinct interfacial adsorption affinities, we demonstrate that depletion interactions and compression-induced instabilities induce domain-selective subduction, a process in which one type of particle domain is driven beneath another. Specifically, more hydrophilic silica domains preferentially collapse and subduct beneath less hydrophilic PS domains, resulting in pronounced vertical asymmetry concentrated at the domain boundaries. Langmuir isotherm analysis and SEM imaging reveal that both the lateral extent of domain segregation and the vertical thickness of the resulting multilayers can be tuned by varying the compression distance and depletant concentration. Lower depletant concentrations reduce depletion pressure, facilitating enhanced particle desorption and enabling the formation of broader and more asymmetric multilayer structures. Importantly, this assembly framework remains effective even when the relative wettability of the particle types is reversed. By introducing sulfonic acid functional groups onto PS, we transform it into a highly hydrophilic species. Adjusting subphase pH to suppress SO₃H dissociation allows both particle types to adsorb at the interface. Under acidic conditions, the PS–SO₃H particles collapse first and subduct beneath silica domains, producing inverted stratification. This inversion confirms that the subduction-driven assembly is not limited to specific wettability pairings, but instead governed by dynamic interfacial energetics and domain interactions.