An adsorption-desorption-controlled surfactant on a deforming droplet
An adsorption-desorption-controlled surfactant on a deforming droplet
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DOI：
10.1006/jcis.1998.5816
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发表时间：
1998-12-01
影响因子：
9.9
通讯作者：
Stebe, KJ
中科院分区：
文献类型：
--
作者：
Eggleton, CD;Stebe, KJ
数值研究了吸附控制的单分子层表面活性剂对液滴在拉伸流场中变形的影响。缩放参数为1厘米和1微米的下降，表明这些结果的适用性。对于所有的模拟，当质量传递是缓慢的相比，表面对流，不溶性的限制恢复;当质量传递是快速的，液滴的行为是相同的，为一个表面无垢液滴。对于形成单层的表面活性剂，表面浓度有一个上限，γ（无穷大）。表面张力减少发散的表面浓度γ接近这个极限，强烈改变流体力学，液滴变形：研究相对于一个无表面活性剂的滴在毛细管数，Ca，特征粘性应力的比率，表面张力。在不溶性的限制，为伽马比伽马（无穷大）小得多，液滴变形比在没有表面活性剂在一个给定的Ca和破碎在较低的Ca。当获得稳定的液滴形状时，在液滴尖端形成停滞帽。有限的表面活性剂的传质速率消除这些帽和减少deformation.For接近伽玛（无穷大）在不溶性极限，界面强烈的扰动表面浓度梯度的压力;伽玛保持几乎均匀的整个变形过程。变形在给定的Ca下减小。当获得稳定的液滴形状时，表面完全停滞。马兰戈尼应力迫使表面速度为零，以保持伽玛低于其上限。对于可溶性表面活性剂，随着传质速率的增加，这些应力的大小减小;变形随传质速率非单调变化，并且不受限制的清洁界面和不溶性limits.The滴贡献的体积平均应力张量σ也计算。轴向分量Sigma（zz）随液滴长度增加;径向分量Sigma（rr）随液滴宽度增加。由于表面活性剂浓度和传质速率强烈影响变形，因此Sigma也是如此。(C)北京：科学出版社.
The effects of a sorption-controlled, monolayer-forming surfactant on a drop deforming in an extensional how are studied numerically. Scaling arguments are presented for drops of 1 cm and 1 pm, indicating the applicability of these results. For all simulations, when mass transfer is slow compared to surface convection, the insoluble limit is recovered; when mass transfer is rapid, the drop behavior is the same as that for a surfactant-free drop. Fora surfactant which forms a monolayer, there is an upper bound to the surface concentration, Gamma(infinity). The surface tension reduction diverges as the surface concentration Gamma approaches this limit, strongly altering the hydrodynamics,The drop deformation is:studied relative to a surfactant-free drop in terms of the capillary number, Ca, the ratio of characteristic viscous stresses to surface tension. In the insoluble limit, for Gamma much less than Gamma(infinity), droplets deform more than in the absence of surfactants at a given Ca and break-up at lower Ca. When stable drop shapes are attained, stagnant caps form at the drop tips. Finite surfactant mass transfer rates eliminate these caps and diminish the deformation.For Gamma approaching Gamma(infinity) in the insoluble limit, interfaces are strongly stressed for perturbative surface concentration gradients; Gamma remains nearly uniform throughout the deformation process. Deformations are reduced at a given Ca. When stable drop shapes are attained, the surface is completely stagnated. Marangoni stresses force the surface velocity to zero to keep Gamma below its upper bound. For soluble surfactants, as mass transfer rates increase, the magnitude of these stresses diminishes; Deformations change nonmonotonically with mass transfer rates and are not bounded by the limiting clean interface and insoluble limits.The drop contribution to the volume averaged stress tensor Sigma is also calculated. The axial component Sigma(zz) increases with the drop length; the radial component Sigma(rr) increases with the drop breadth. Since the deformation is strongly influenced by the surfactant concentration and the mass transfer rates, so too is Sigma. (C) 1998 Academic Press.