Investigating secondary nucleation at the microscale

晶体的二次成核是制药工艺和结晶很重要的其他应用和自然现象中普遍存在的现象。已经形成的初级晶核响应于不同范围的刺激（例如机械扰动、来自流动的剪切应力和杂质的存在）而产生新的次级晶核。在制造中，不受控制的二次成核可能导致难以管理关键工艺变量和结果参数，包括晶体固体形式、生长速率和粒度。二次成核已被证明是微妙的敏感条件，包括轻微的变化，机械扰动和流动，提出了重大的挑战，应用。这种敏感性的根本原因必须在于分离和添加分子材料从现有的核和现有的核的微观过程中，但很少有深入了解这些过程如何影响成核。大多数关于二次成核的工作都集中在宏观本体响应上，例如对流动和搅拌的响应，以及该过程如何扩展到工业相关的体积，例如在制药中从晶种产生二次晶体。目前的许多行业知识和战略都是基于这些经验和试错法，并受到这些经验和试错法的限制。这个项目将采取直接的微观尺度的方法，以提高我们的理解如何微尺度的机械扰动和剪切流影响二次成核。我们建议使用一种新的方法的基础上捕获的颗粒和机动的光镊，以提供受控的微米级的机械干扰的晶种作为源的二次成核。直接光学观察，然后提供数据形成的二次晶核，包括成核和生长速率和空间分布的新晶体相对于种子和干扰位置。通过控制扰动力、扰动速率和扰动类型，（例如对晶种的“直接”或掠射冲击，或“刮”干扰粒子沿着晶种表面），然后我们可以研究微米级事件如何触发二次成核以及机械因素如何影响随后的晶体生长。（他的发明为阿特·阿什金赢得了2018年诺贝尔物理学奖）使用聚焦激光束捕获悬浮在液体中的微米级探针粒子。捕获光束的转向可用于操纵探针并以受控方式施加局部力。此外，探针还充当测量装置：镊子的所谓“被动”模式可用于通过跟踪阱内的颗粒位置并从光学阱的谐波强度推断局部力平衡来测量由于材料从种子释放而引起的悬浮介质的局部粘度和密度的变化。因此，通过交替地操纵粒子以产生扰动，随后跟踪“被动”粒子以测量粘度和密度的局部变化，我们将联合收割机扰动器和测量器组合在一个设备中。该实验在光学显微镜中进行，该光学显微镜提供捕获/转向光束的聚焦、颗粒的位置跟踪、成像以揭示指示早期晶体成核的光学对比度（折射率）的局部变化，以及一旦次级核大于约5-10微米并且可以光学分辨，则直接观察包括生长形状的后期晶体生长。为了开发进一步的定量解释的基础上施加的力的响应，我们还将进行流体动力学建模的扰动流场，使用最近开发的内部流体动力学分子动力学代码。最后，我们还将探讨详细的分子模拟的晶体种子表面的外力响应的潜力

Secondary nucleation of crystals is a ubiquitous phenomenon in pharmaceutical manufacturing processes and other applications and natural phenomena where crystallization is important. Already formed primary crystal nuclei generate new secondary nuclei in response to a varied range of stimuli such as mechanical disturbance, shear stress from flow and the presence of impurities. In manufacturing uncontrolled secondary nucleation can lead to difficulties managing key process variables and outcomes parameters including crystal solid form, growth rates and particle sizes. Secondary nucleation has been shown to be exquisitely sensitive to conditions including slight variations in mechanical disturbance and flow, presenting significant challenges for applications. The fundamental reasons for this sensitivity must lie in microscopic processes of detachment and addition of molecular material from and to existing nuclei, but there is little deep understanding of how these processes influence nucleation. Most work on secondary nucleation has focussed on macroscopic bulk response eg to flow and agitation, as well as how the process then scales up to industry-relevant volumes, for example in secondary crystal generation from seed crystals in pharmaceutical manufacture. Much of the current industry knowhow and strategy is based on and limited by these empirical and trial-and-error approaches. This project will instead take a direct microscopic scale approach to improve our understanding of how microscale mechanical disturbance and shear flow influence secondary nucleation. We propose using a novel approach based on a particle trapped and manoeuvred by an optical tweezer to provide controlled micron-scale mechanical disturbance of a seed crystal acting as the source of secondary nucleation. Direct optical observation then provides data on formation of secondary nuclei including nucleation and growth rates and spatial distribution of new crystals relative to seed and disturbance location. By controlling disturbance force, and rate and type (eg 'direct' or glancing impact on the seed crystal, or 'scraping' the disturber particle along the seed surface) we can then investigate how micron-scale events trigger secondary nucleation and how mechanical factors influence later crystal growth.Optical tweezers (whose invention gained Art Ashkin a share of the 2018 Nobel Prize in Physics) use a focussed laser beam to trap a micron-sized probe particle suspended in a liquid. Steering of the trapping beam can be used to manipulate the probe and apply local forces in a controlled way. Furthermore the probe also acts as measuring device: so-called 'passive' mode of the tweezer can be used to measure changes in local viscosity and density of the suspending medium due to release of material from the seed, by tracking the particle position within the trap and inferring from the harmonic strength of the optical trap the local force balance. Therefore by alternately steering the particle to create a disturbance, and subsequently tracking the 'passive' particle to measure local change in viscosity and density, we combine disturber and measurer in one device. The experiment is carried out in an optical microscope, which provides focussing of the trapping/steering beam, position tracking of the particle, imaging to reveal local changes in optical contrast (refractive index) indicating early stage crystal nucleation, and direct observation of later stage crystal growth including growth shapes, once secondary nuclei are greater than around 5-10 microns and can be optically resolved. In order to develop further quantitative interpretation of the response based on the applied forces, we will also undertake hydrodynamic modelling of the disturbed flow field, using recently developed in-house hydrodynamic molecular dynamics codes. Finally we will also explore the potential for detailed molecular modelling of the response of the crystal seed surface to external force