Deformable particles with anisotropic interactions: unusual field-induced structural transitions in ultrasoft ionic microgel colloids

Ionic microgels are intriguing soft and deformable colloids with an effective pair potential that crosses over from Yukawa-like at large distances to a much softer repulsive interaction at short distances. Here we report the effect of adding an anisotropic dipolar contribution to colloids with such "ultra-soft" interactions. We use an alternating electric field to induce a tunable dipolar contribution, and study the resulting particle self-assembly and phase transitions in situ with confocal laser scanning microscopy. We find significant field-induced structural transitions at low as well as at very high effective volume fractions. At phi(eff) = 0.1 we observe a transition from an isotropic to a string fluid. At phi(eff) = 0.85, there is a reversible transition from an amorphous to a dipolar crystalline state, followed by the onset of a gas-(string) solid coexistence. At phi(eff) = 1.6 and 2.0, i.e. far above close packing, evidence for a field-induced arrested phase separation is found.

离子微凝胶是一种有趣的柔软且可变形的胶体，其有效对势在远距离时从类汤川势转变为近距离时更软的排斥相互作用。在此，我们报道了在具有这种“超软”相互作用的胶体中添加各向异性偶极贡献的影响。我们利用交变电场诱导出一种可调的偶极贡献，并通过共聚焦激光扫描显微镜原位研究由此产生的粒子自组装和相变。我们发现在低有效体积分数以及非常高的有效体积分数下都存在显著的场诱导结构转变。在有效体积分数\(φ_{eff}=0.1\)时，我们观察到从各向同性到链状流体的转变。在\(φ_{eff}=0.85\)时，存在从无定形到偶极晶态的可逆转变，随后出现气 -（链状）固共存。在\(φ_{eff}=1.6\)和\(2.0\)（即远高于密堆积）时，发现了场诱导的阻滞相分离的证据。