Liqun He

Size-dependent exchange bias in half-doped manganite nanoparticles

Magnetic properties of the nanosized half-doped manganite of Sm0.5Ca0.5MnO3 with different particle sizes prepared by a sol-gel method are investigated. Exchange-bias phenomena are observed in the field-cooled magnetic hysteresis loops for these nanoparticles. The values of the exchange field, coercivity, remanence asymmetry, and remanent magnetization are found to depend strongly on the particle size. Particularly, as the particle size decreases, the exchange field shows a nonmonotonic variation with a maximum at ∼120 nm. These magnetic behaviors have been explained in terms of size effects on the charge ordered and antiferromagnetic manganite nanoparticles.

Size-induced transition from non?Griffiths-like to Griffiths-like clustered phase above the Curie temperature

Recent studies on several pervoskite oxides such as Sm0.5Sr0.5MnO3, La0.7Sr0.3CoO3, and La2NiMnO6, reveal that their inverse susceptibilities exhibit an upward deviation from Curie-Weiss behavior above TC, indicating the existence of a non–Griffiths-like clustered phase due to the dominance of antiferromagnetic interactions in this regime. Here, we study the magnetic behaviors of their nanoparticles and find that the deviation becomes sharply downward, which is a typical characteristic of a Griffiths-like phase. This suggests that reducing the particle size induces a phase transition from non–Griffiths-like to Griffiths-like. We propose that the weakening of the antiferromagnetic interactions by size reduction is responsible for this transition.

Rapid fabrication of microfluidic chips based on the simplest LED lithography

Microfluidic chips are generally fabricated by a soft lithography method employing commercial lithography equipment. These heavy machines require a critical room environment and high lamp power, and the cost remains too high for most normal laboratories. Here we present a novel microfluidics fabrication method utilizing a portable ultraviolet (UV) LED as an alternative UV source for photolithography. With this approach, we can repeat several common microchannels as do these conventional commercial exposure machines, and both the verticality of the channel sidewall and lithography resolution are proved to be acceptable. Further microfluidics applications such as mixing, blood typing and microdroplet generation are implemented to validate the practicability of the chips. This simple but innovative method decreases the cost and requirement of chip fabrication dramatically and may be more popular with ordinary laboratories.

Controllable geometry-mediated droplet fission using “off-the-shelf” capillary microfluidics device

We describe “off-the-shelf” capillary microfluidic devices for droplet fission. The cheap and simple commercial capillary adapters are characterized by perfect sealing, easy assembly and convenient cleaning. Primary droplets, bigger than 65 μm, without being plugs, can be split into uniform daughter droplets (15–170 μm) to obtain smaller droplets and increase their concentration. The relative size of daughter droplets can be easily controlled by adjusting the length of the glass capillary tube.

The instability of monodisperse bubbles passing through a confined geometry

The dispersed bubble experiences an impact of flow-focusing from the outer viscous liquid, and may break into satellite bubbles after flowing through the narrower section. We show that the number of satellite bubbles can be characterized by a phase diagram that depends on the capillary number and the Weber number. The number of satellite bubbles can be estimated through the Tgrow (the growth time of the daughter bubble), Tpinch (the pinch-off time of the daughter bubble), and Tpass (the time the initial microbubble need to pass the pore). We defined N = Tpass/(Tpinch + Tgrow) as the dimensionless time and the critical condition: N ∼ 1 to evaluate the bubble breakup. Our work shows that the breakup of the microbubble or droplet through a sudden narrowing joint is predictable and controllable.