在稀薄的膠體溶液中，膠體粒子受到重力和熱擾動互相競爭的影響，而達到沉降平衡，形成一個非均勻的濃度分佈，此濃度分佈會遵守大氣分佈(barometric distribution)。然而，即使在低體積分率下(例如：10-4)，帶電粒子之間的靜電作用也會造成其濃度分佈與大氣分佈有著重大的偏差。在本研究中，藉由蒙地卡羅模擬(Monte Carlo simulations)方法來模擬在稀薄、未加鹽的膠體溶液系統中，沉降平衡的濃度分佈。發現在沉降平衡系統中因其濃度的分佈，可以粗略的區分為五個部份，而分別可以從這五個部份的膠體粒子和反離子的濃度分佈中，觀察反離子凝聚效應的程度。進而從模擬結果中得知造成Non-barometric distribution的原因主要來自於膠體粒子彼此之間的斥力作用，而和膠體粒子與反離子之間的吸引力無關。再進一步觀察反離子的濃度分佈發現，當膠體粒子與反離子之間為弱吸引力時，反離子在系統溶液中均勻分佈，並隨著彼此之間吸引力增加的同時，反離子會逐漸吸附在膠體粒子週遭。進一步發現，每當膠體粒子彼此之間為弱斥力作用時，膠體粒子的濃度分佈遵守大氣分佈。整體而言，在Non-barometric distribution情況下，溶液的局部並不會達到電中性，而內部電場的存在並不是造成Non-barometric distribution的主要原因。 The sedimentation profile of a dilute colloidal solution follows the barometric distribution owing to the balance between gravitational force and thermal fluctuation. However, the electrostatic interaction between charged particles may lead to significant deviation even in the low volume fraction limit (e.g. 10-4). On the basis of Monte Carlo simulations for a dilute, salt-free colloidal dispersion, five regimes in terms of the degree of counterion condensation can be identified by the sedimentation profile and counterion distribution. The non-barometric distribution is principally caused by strong colloid-colloid repulsion irrespective of colloid-ion attraction. On the other hand, the counterion distribution is uniform at weak colloid-ion attraction and becomes to accumulate in the vicinity of the colloids at strong colloid-ion attraction. Nonetheless, both cases lead to barometric profile if colloid-colloid repulsion is weak. Local electroneutrality condition is generally not satisfied and charge separation (or internal electric field) is neither a sufficient nor necessary condition for non-barometric distributions.
