离心技术在生物科学，特别是在生物化学和分子生物学研究领域中，已经得到了十分广泛的应用。每个生物化学和分子生物学实验室都要装备多种型式的离心机。离心技术用于各种生物样品的分离和制备，主要有两个目的：在离心力（centrifugal force）的作用下浓缩 (concentrate) 和提纯 (purify) 想要的物质，而沉降速度则取决于颗粒的质量、大小和密度等等因素。
在这篇文章中，小编会给大家主要讲解一下超速离心（ultracentrifugation）的主要物理原理 （physics of centrifugation）。首先，大家试想下一颗下沉粒子（sinking particle）所受到的力。根据斯托克斯定理 （Stoker’s Law），一个下落球体 (falling sphere) 的沉降速度（sedimentation velocity, v (ms^-1)）是 (Lawrence & Steward 2010)：
d：粒子的直径 （m）
ρ_p：粒子的密度 （kg m^-3）
ρ_m：液体介质的密度（kg m^-3）
g：重力加速度（m s^-2）
μ：液体的粘性（Pa s）
在超速离心的情况下，离心力远远大于重力，从而取代了重力的表述，公式因此变成了以下这个形式：
ω:角速度 （radians s^-1）
r: 颗粒到旋转轴的距离 （m）
通过这个公式我们可以看出，一颗粒子在超速离心的时候主要受到两个力的影响: 浮力（buoyancy：ρ_p-ρ_m）和下沉力（sedimenting forces：ω^2 r）。这颗粒子的下沉速度同时还受到公式中提到的液体粘度和粒子大小的影响。但总的来说，这两个力的相互作用影响了粒子的下沉状态。
通常情况下，粒子的密度是一个很重要的系数。与此同时，一个不是很显而易见但是非常重要的因素也要被纳入考虑之中，那就是沉降系数（sedimentation coefficient），以下是它的计算公式：
从公式中大家能看出来，沉降系数就是一个粒子的沉降速度与使其沉降的加速度的比率。在现代表述中，沉降系数有一个特定的时间单位，这个单位是斯维德伯格Svedbergs (S)，被精准的定义为10-13 s。本质上，由于离心力越大离子沉降越快，沉降系数是用离心施加的加速度大小平衡颗粒的沉降速率。这样所得到的值就不再依赖于加速度大小，而仅由颗粒的性质和它悬浮介质决定。
当同时考虑到密度和沉降系数时，就可以比较病毒颗粒或是其他生物颗粒的沉降特点了 （图表1）。这样一来，就可以选用最有效的离心方法来进行浓缩、纯化。在文献引用中，沉降系数通常是在水中20℃时的值。
图表1：亚细胞体（subcellular entities）的密度、直径和沉降系数。
小编会在下一篇文章给大家介绍一下不同种类的离心方法，欢迎大家积极留言参与讨论！
Referecnes
Lawrence, J.E. & Steward, G.F., 2010. Purification of viruses by centrifugation. Manual of Aquatic Viral Ecology. ASLO, 2(2), pp.166–181.
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