by Erica K.
Brockmeier
發表在《自然通訊》(Nature Communication)的新研究發現一堆沙子即使是在未受擾動的情形下仍會不停移動。透過極為敏銳的干涉光所得到的數據，賓州大學與范德堡大學的研究人員得出的結論挑戰了地質學與物理學之中，關於土壤以及其他種類的無序材料會有什麼行為的現有理論。
大多數的人們只有在山丘上的土壤突然軟化時才注意到它們會運動，這種現象稱為「降伏」(yield)。「假設你在山坡上面堆積一些砂土，之後如果有地震發生或是下雨，這些看似固體的物質就會變得像液體一樣，」研究主持人，賓州大學的Douglas
Jerolmack表示。「目前的主流理論把其視為一種由裂隙造成的破壞。這會產生問題的原因在於模擬這種材料的時候依循的準則是固體力學，但卻是在模擬它們變成液體的那瞬間，因此從本質上來說就是矛盾的。」
這類模型意謂土壤還沒降伏時會是固體，因此不該有流動的行為，然而土壤在降伏點以下便會經由潛移(creep)
作用而緩慢地持續「流動」。地質學裡的主流解釋認為土壤潛移的原因是物理或生物作用造成的擾動，像是水分反覆地凍結融化、樹木倒塌、動物挖土…...等讓土壤移動的行為。
賓州大學的博士候選人Nakul
S. Deshpande是研究主要作者，他的研究興趣是觀察靜止狀態下個別沙粒的運動。根據現有的理論，它們應該是完全不動的。「之前有些研究人員假設潛移中的沙粒會有某些行為而建立了模型，但實際上卻沒有人直接去觀察沙粒的動作，」Deshpande表示。
為此Deshpande策畫了一連串看似簡單的實驗。他在可以隔絕震動的工作台上放置小型的壓克力箱，然後在裡面倒出沙堆。接著他用稱為擴散波光譜法(diffusing-wave
spectroscopy)的雷射光散射技術來進行觀察，這種方法對於非常微小的顆粒的動作十分敏銳。「從技術上來說這項實驗相當困難，」Deshpande談論這項研究時表示。「該技術在物理學中很少見到需要如此高解析度的情況；而在地球科學或地形學中還沒有用到它的前例。」
在Jerolmack的實驗室，他們利用擴散波光譜法來研究沙堆中的顆粒非常細微的運動(圖版左邊)。收集到的數據以應變率分布圖來呈現(圖版右邊)，結果顯示在未受擾動的情形下，經過11天之後顆粒仍在持續運動。圖片來源：Nakul
Deshpande
與Deshpande和Jerolmack一起進行這項研究的還有他們長期合作的夥伴，賓州大學複雜流體實驗室的主持人
Paulo Arratia。他們從物理學、材料科學和工程學的背景來探討他們的實驗數據，藉此找出可供類比的系統與理論來解釋他們的結果。范德堡大學的David
Furbish則提出了解釋，說明為什麼過往的模型在物理上並不適用，而且跟研究人員的發現會不一致。Furbish的專長是利用統計物理學來研究粒子的運動如何影響大尺度的地形演變。
第一項實驗看似十分簡單：在盒子中倒出一堆沙，靜置後用雷射觀察。雖然從直覺上來想，以及主流理論都認為未受擾動的沙堆應該會是靜止的，但是研究人員卻發現沙堆實際上是會不斷運動，就像玻璃一樣的物質。
「我們對於沙子能進行的所有測量結果，都顯示它們就像冷卻中的玻璃一樣柔軟，」Deshpande表示。「如果把一個玻璃瓶融化之後再降溫，當玻璃冷卻時裡面的分子的行為，從我們能運用的所有測量方法來看，都跟沙子沒有分別。」
玻璃和土壤顆粒在物理學中皆為典型的「無序」系統，它們的組成顆粒是隨機排列的，這和結構明確的結晶質並不相同。而賓州大學材料科學及工程研究中心的重點領域之一便是無序材料。雖然無序材料在受到壓力變形時會有一些共同的行為，但是玻璃和沙堆之間還是有一項重要的差異：組成玻璃的分子會一直隨機移動，其速率由溫度來決定；但是沙粒的體積太大而無法辦到這點。因為如此，物理學家預期一堆沙子應該會彼此「堵塞」而無法移動，但是這項最新的發現卻讓物理學和地質學的研究人員能用新的方式來看待土壤。
另一項驚人的發現是土壤潛移的速率可以經由擾動的種類來控制。雖然未受擾動的沙堆在研究人員的觀察期間會一直潛移，但是顆粒的移動速度會經由「老化」的過程而逐漸變慢。不過沙粒受熱之後，便能逆轉老化作用而讓潛移速率提升回起始值。反之，拍打沙堆則會加速老化過程。
「我們比較常想到讓土壤趨向降伏的事物，像是地震造成的晃動可能會造成山崩，但是自然界的其他擾動卻有可能避免土壤降伏，也就是讓山崩變得不易發生，」Jerolmack表示。「Nakul顯示他可以任意地把沙堆推向或是遠離降伏點的時候，就像是顆炸彈在我們面前爆炸一樣，這是個完全嶄新的領域。」
研究人員的近期目標是進行更加深入的實驗，他們計畫用磁針來重現局部擾動的影響，藉此了解擾動如何讓一個系統趨向或者遠離降伏。此外，他們也正在檢視實地觀測的數據，從自然的土壤潛移到造成災難的崩塌事件，目的是探討他們的實驗室試驗是否能連結至實地觀察到的現象，這或許可以讓研究人員有新的方法來事前偵測出大規模的地貌破壞事件。
現有理論依據的思考模式通常像是「有座山丘，上面的土壤會隨著時間移動……」，研究人員希望他們的成果是個起點，使得這種思維能被拋棄而讓理論得以改良。「當你觀察到違反直覺的全新現象，要讓它成為人們使用的模型得花上很長的時間，」Jerolmack說。「我希望地球科學領域之中擁有先進工具技術以及經驗老到的科學家，會接續我們研究結束的地方然後說：『我有個你們完全想像不到的全新想法，可以在野外找尋這些訊號』――如此一來，這個領域的規模、能力，以及對它的興趣便能自然而然的傳承下去。」
Shifting sands, creeping soils, and a
new understanding of landscape evolution
A new study published in Nature Communications finds that piles
of sand grains, even when undisturbed, are in constant motion. Using
highly-sensitive optical interference data, researchers from the University of
Pennsylvania and Vanderbilt University present results that challenge existing
theories in both geology and physics about how soils and other types of
disordered materials behave.
Most people only become aware of soil movement on
hillsides when soil suddenly loses its rigidity, a phenomenon known as yield.
“Say that you have soil on a hillside. Then, if there’s an earthquake or it
rains, this material that’s apparently solid becomes a liquid,” says principal
investigator Douglas Jerolmack of Penn. “The prevailing framework treats this
failure as if it’s a crack breaking. The reason that’s problematic is because
you’re modeling the material by a solid mechanical criterion, but you’re modeling
it at the point at which it becomes a liquid, so there’s an inherent
contradiction.”
Such a model implies that, below yield the soil is a
solid and therefore should not flow, but soil slowly and persistently “flows”
below its yield point in a process known as creep. The prevailing geological
explanation for soil creep is that it is caused by physical or biological
disturbances, such as freeze-thaw cycles, fallen trees, or burrowing animals,
that act to move soil.
In this study, lead author and Penn Ph.D. candidate
Nakul S. Deshpande was interested in observing individual sand particles at
rest which, based on existing theories, should be entirely immobile.
“Researchers have built models by presuming certain behaviors of the soil
grains in creep, but no one had actually just directly observed what the grains
do,” says Deshpande.
To do this, Deshpande set up a series of seemingly
simple experiments, creating sand piles in small plexiglass boxes on top of a
vibration isolation worktable. He then used a laser light scattering technique
called diffusing-wave spectroscopy, which is sensitive to very small grain
movements. “The experiments are technically challenging,” Deshpande says about
this work. “Pushing the technique to this resolution is not yet common in
physics, and the approach doesn’t have a precedent in geosciences or
geomorphology.”
Deshpande and Jerolmack also worked with long-time
collaborator Paulo Arratia, who runs the Penn Complex Fluids Lab, to connect
their data with frameworks from physics, materials science, and engineering to
find analogous systems and theories that could help explain their results.
Vanderbilt’s David Furbish, who uses statistical physics to study how particle
motions influence large-scale landscape changes, provided explanation for why
previous models were physically inadequate and inconsistent with what the
researchers had found.
The first experiments were seemingly easy: Pour a
pile of sand into the box, let it sit, and watch with the laser. But the
researchers discovered that, while intuition and prevailing theories say that
the undisturbed piles of sand should be static, sand grain piles are in fact a
mass of constant movement and behave like glass.
“In every way that we can measure the sand, it is
relaxing like a cooling glass,” says Deshpande. “If you were to take a bottle
and melt it, then freeze it again, that behavior of those molecules in that
cooling glass are, in every way that we’re capable of measuring, just like the
sand.”
In physics, glass and soil particles are classic
examples of a “disordered” system, one whose constituent particles are arranged
randomly instead of in crystalline, well-defined structures. While disordered
materials, a major focus area of Penn’s Materials Research Science &
Engineering Center, share some common behaviors in terms of how they deform
when stressed, there is an important difference between glass and a pile of
sand. The molecules that make up glass are always moving randomly at a rate
that depends on temperature, but sand grains are too large to do that. Because
of that, physicists expect that a pile of sand would be “jammed” and unmoving,
but these latest findings present a new way of thinking about soil for
researchers in both physics and geology.
Another surprising result was that the rate of
creeping soil could be controlled based on the types of disturbances used.
While the undisturbed sandpile continued to creep for as long as the
researchers observed, the rate of particle motion slowed through time in a
process called aging. When sand particles were heated, this aging was reversed
such that creep rates increased back to their initial value. Tapping the pile,
in contrast, accelerated aging.
“We tend to think of things that drive soil toward
yield, like shaking from an earthquake that triggers a landslide, but other
disturbances in nature potentially drive soil further away from yield, or make
it harder for a landslide to happen,” says Jerolmack. “Nakul’s ability to tune
it further or closer to yield was like a bomb that went off for us, and this is
an all-new area.”
In the near term, the researchers are working on
follow-up experiments to recreate the impacts of localized disturbances using
magnetic probes to understand how disturbances could lead a system further away
from or closer to yield. They are also looking at data from field observations,
from natural soil creep to catastrophic landslide events, to see if they can
connect their lab experiments to what observers see in the field, potentially
enabling new ways to detect catastrophic landscape failures before they happen.
The researchers hope that their work can be a
starting point for refining existing theories that rely on a paradigm that,
like a hillside whose soil particles have shifted over time, no longer holds
weight. “When you observe something really counterintuitive and new, it’s going
to now take a long time before that turns into a model to use,” says Jerolmack.
“I hope on the geoscience side that people with sophisticated tools and
techniques and experience will pick up where we’ve ended and say, ‘I have a new
idea for seeking this signature in the field that you wouldn’t have thought
of’—that natural handoff of scales and abilities and interests.”
原始論文：Nakul S.
Deshpande, David J. Furbish, Paulo E. Arratia, Douglas J. Jerolmack. The
perpetual fragility of creeping hillslopes. Nature Communications,
2021; 12 (1) DOI: 10.1038/s41467-021-23979-z
引用自：University of Pennsylvania. "Shifting
sands, creeping soils, and a new understanding of landscape
evolution."
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