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1. A Surfactant-Free Microfluidic Process For Fabrication Of Multi-Hollow Polyimide Aerogel Particles

   Yao, Y. ; Joo, P. ; Jana, S.C. ( October 2020 , International polymer processing)

   This work focuses on fabrication of multi-hollow polyimide gel and aerogel particles from a surfactant-free oil-in-oil emulsion system using a microfluidic droplet generator operating under dripping mode. The multi-hollow gel and aerogel particles have strong potential in thermal insulation. Under jetting and tip-streaming regime of microfluidic flows, droplets are generated with no occluded liquid phase. The present study investigates a means of designing polyimide gel particles with plurality of internal liquid droplets by strategically manipulating the flow rates of the continuous and dispersed phase liquids through the microfluidic droplet generator. The multi-hollow polyimide aerogel particles obtained after supercritical drying of the gel particles present mesopores, high BET surface area, and excellent prospect for thermal insulation. 

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2. Electro-elastic migration of particles in viscoelastic fluid flows

   https://doi.org/10.1063/5.0167571  

   Li, Di ; Xuan, Xiangchun ( September 2023 , Physics of Fluids)

   Microfluidic manipulation of particles usually relies on their cross-stream migration. A center- or wall-directed motion has been reported for particles leading or lagging the Poiseuille flow of viscoelastic polyethylene oxide (PEO) solution via positive or negative electrophoresis. Such electro-elastic migration is exactly opposite to the electro-inertial migration of particles in a Newtonian fluid flow. We demonstrate here through the top- and side-view imaging that the leading and lagging particles in the electro-hydrodynamic flow of PEO solution migrate toward the centerline and corners of a rectangular microchannel, respectively. Each of these electro-elastic particle migrations is reduced in the PEO solution with shorter polymers though neither of them exhibits a strong dependence on the particle size. Both phenomena can be reasonably explained by the theory in terms of the ratios of the forces involved in the process. Decreasing the PEO concentration causes the particle migration to shift from the viscoelastic mode to the Newtonian mode, for which the magnitude of the imposed electric field is found to play an important role.

    

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3. Effect of Filler Aspect Ratio on Stiffness and Conductivity in Phase‐Changing Particulate Composites

   https://doi.org/10.1002/admt.202100920  

   Mohammadi Nasab, Amir ; Buckner, Trevor L. ; Yang, Bilige ; Kramer‐Bottiglio, Rebecca ( October 2021 , Advanced Materials Technologies)

   Variable stiffness in elastomers can be achieved through the introduction of low melting point alloy particles, such as Field's metal (FM), enabling on‐demand switchable elasticity and anisotropy in response to thermal stimulus. Because the FM particles are thermally transitioned between solid and liquid phases, it is beneficial for the composite to be electrically conductive so the stiffness may be controlled via direct Joule heating. While FM is highly conductive, spherical particles contribute to a high percolation threshold. In this paper, it is shown that the percolation threshold of FM particulate composites can be reduced with increasing particles aspect ratio. Increasing the aspect ratio of phase‐changing fillers also increases the rigid‐to‐soft modulus ratio of the composite by raising the elastic modulus in the rigid state while preserving the low modulus in the soft state. The results indicate that lower quantities of high aspect ratio FM particles can be used to achieve both electrical conductivity and stiffness‐switching via a single solution and without introducing additional conductive fillers. This technique is applied to enable a highly stretchable, variable stiffness, and electrically conductive composite, which, when patterned around an inflatable actuator, allows for adaptable trajectories via selective softening of the surface materials.

    

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4. Capillary induced twisting of Janus cylinders

   https://doi.org/10.1039/c7sm01288h  

   Oratis, Alexandros T. ; Farmer, Timothy P. ; Bird, James C. ( January 2017 , Soft Matter)

   At the appropriate length scales, capillary forces exerted by a liquid in contact with a compliant solid can cause the solid's deformation. Capillary forces are also able to align particles with discrete wettabilities – or Janus particles – at liquid interfaces. Their amphiphilic properties enable Janus particles to orient themselves at liquid interfaces such that both of their surfaces are facing their preferred fluid. However, it is unclear how to spontaneously obtain varying degrees of rotational alignment. Here we extend ideas of elasto-capillarity to modulate rotational alignment by connecting amphiphilic Janus cylinders in an antisymmetric configuration. As the Janus cylinders rotate they cause a twisting deformation of rod. We develop both a mathematical model and a physical macroscale setup to relate the angle of twist to the elastic and interfacial properties, which can be used to tune the extent of alignment of Janus particles at air–water interfaces. We additionally extend our analysis to calculate the twist profile on a compliant element with a distributed capillary torque. 

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5. Sculpting stable structures in pure liquids

   https://doi.org/10.1126/sciadv.aav4283  

   Emeršič, Tadej ; Zhang, Rui ; Kos, Žiga ; Čopar, Simon ; Osterman, Natan ; de Pablo, Juan J. ; Tkalec, Uroš ( February 2019 , Science Advances)

   null (Ed.)

   Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized optothermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled nonequilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli. 

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