「サブミクロン粒子の離脱」の難関を低出力型超音波洗浄機で突破できるか?Can a low-power ultrasonic cleaner overcome the challenge of removing submicron particles?

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100 nm から1μm(Sub-micron)粒子は大気中に浮遊する物質の中で最も高い個数濃度であり、その観測や化学分析等が気候変動の予測に不可欠。化学分析を行う際に、しばしば大気中微粒子を固体表面(フィルター等)に捕集した後に液体へ転換させる。しかし、固体表面上に堆積したsub-micron粒子を高い効率で離脱させることが難しい課題となっている。

本研究では、目的のsub-micron粒子が堆積された基板に対して、100~200nm(平均サイズ)粒子が含まれている水系サスペンションに超音波法を利用した粒子回収法(洗浄法)が提案された。サンプルとして,燃焼由来のsubmicron酸化マグネシウム(MgO)の凝集体(粒子層)が堆積された平板(シリコンウエハ,粗いアルミナコート樹脂シート、細かいアルミナコート樹脂シート)を使用した。サスペンションに添加したのはロウソク燃焼由来の煤粒子(soot)。出力が低い(42kHz,35W)超音波洗浄機を用いて離脱・剥離実験を試みた。

電子顕微鏡(SEM)画像から、煤を含む媒体を用いた離脱・剥離実験は、煤を含まない媒体を用いた場合と比較して高い効率を示し、微粒子の離脱・剥離に最適な煤の濃度があることもわかった。超音波処理後の懸濁液の粒度分布から、サンプル粒子が煤の懸濁液中に良好に分散していることがわかった。

100 nm to 1 μm (sub-micron) particles are said to have the highest number concentrations of any airborne material. In order to perform detailed chemical analysis, it is often necessary to collect atmospheric particles on a filter (solid substrate) and transfer them to a liquid phase. However, it has been a difficult task to release the sub-micron particles deposited on the solid surface with high efficiency.

A protocol for the detachment of submicron (100-1000 nm) particulate samples deposited on flat substrates and their collection in aqueous samples is proposed based on a suspension-assisted ultrasonic method. As samples, combustion-made magnesium oxide (MgO) aggregates in the submicron size range were deposited in the gas phase onto three kinds of substrates: a silicon wafer and coarse and fine alumina-coated resin sheets. To enhance the sample particle detachment, a solid–liquid suspension made of candle combustion soot particles (having average sizes between 100-200 nm) was selected as an ultrasound propagation medium, which is different from the usual liquid medium, such as water, a surfactant solution, or a solvent.

Experiments were performed using low-power (42kHz and 35W) ultrasonication, and the substrates and suspensions were analyzed using scanning electron microscopy (SEM) images and particle size distributions based on dynamic light scattering (DLS), respectively.

The detachment using the medium with soot had a higher efficiency compared to that without soot, and there was an optimum soot concentration for particle detachment for all three substrates. The suspension particle size distribution after ultrasonication showed good dispersion of the sample particles in the soot suspension.

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Short Introduction of our other papers – 研究室が発表した論文の紹介 (empatlab.net)