Mapping and controlling nucleation

This thesis reports on an investigation of two non-trivial nucleation phenomena, with the goal that understanding these will enable greater understanding of nucleation generally. Non-photochemical laser-induced nucleation (NPLIN) phenomena have been investigated for more than twenty years but lack a complete explanation. The laser-induced phase separation and nucleation (LIPSaN) phenomenon has been discovered and it is proposed as the mechanism behind NPLIN. A laser generates a potential which, when incident on a binary mixture in proximity to its critical point, causes the high-refractive index component to migrate to the focus. The effect bears a similarity to optical trapping of particles, as the trapped particle has a higher refractive index than the medium it is in. It has been shown that nucleation can be induced in metastable binary mixtures, which is analogous to the metastable supersaturated solutions which are typical in NPLIN experiments. It is proposed that NPLIN can only work if there is a hidden liquid-liquid critical point in the supersaturated regime. The liquid-liquid transition (LLT) is a ubiquitous example of polyamorphism – the transition between one liquid state with no long-range ordering to another. There are several examples of LLTs, but none that are quite as fiercely debated as triphenyl phosphite (TPP). The debate can be summarised as two competing hypotheses: Hédoux – the second liquid does not exist; it is actually the untransformed liquid mixed with nano or micro scale crystals, or Tanaka – the second liquid state exists, but nano or micro crystals are also produced at higher LLT temperatures. It will be shown using a wide range of techniques that Tanaka is at least partially correct, but the two sides are two sides of the same coin. The second liquid state exists and there is a first order LLT, but the so-called nanocrystals are better described as locally favoured structures that are similar to the structure of the crystal. There are three crystal polymorphs of TPP and their distinct unit cells and conformers have been characterised with single crystal X-ray diffraction (XRD). There is a conformational change during the LLT which has been characterised using infrared, density functional theory (DFT) and XRD data. The LLT is associated by a flip of a phenoxy arm and change from parallel ‘sandwich’ to T-shaped π- π stacking. Both avenues of investigation emphasise the importance of critical points and their influence in how nucleation proceeds. The work presented here sheds some light on two poorly understood nucleation phenomena and will hopefully aid in a more robust understanding of nucleation generally

Control Over Phase Separation and Nucleation Using a Optical-Tweezing Potential

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a black art. Although there are now numerous examples of control using laser-induced nucleation, a physical understanding is absent and preventing progress. We will show that concentration fluctuations in the neighborhood of a liquid-liquid critical point can be harnessed by an optical-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in liquid mixtures using a low-power laser diode confirm the effect. Phase separation and nucleation through an optical-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter

Frustration of crystallisation by a liquid–crystal phase

Frustration of crystallisation by locally favoured structures is critically important in linking the phenomena of supercooling, glass formation, and liquid-liquid transitions. Here we show that the putative liquid-liquid transition in n-butanol is in fact caused by geometric frustration associated with an isotropic to rippled lamellar liquid-crystal transition. Liquid-crystal phases are generally regarded as being “in between” the liquid and the crystalline state. In contrast, the liquid-crystal phase in supercooled n-butanol is found to inhibit transformation to the crystal. The observed frustrated phase is a template for similar ordering in other liquids and likely to play an important role in supercooling and liquid-liquid transitions in many other molecular liquids

Conversational shaping: staff-members' solicitation of talk from people with an intellectual impairment

In initiating and maintaining talk with people with intellectual impairments, members of care staff use a range of recurrent conversational devices. The authors list six of the more common of these devices, explain how they work interactionally, and speculate on how they serve institutional interests. As in other dealings between staff members and the people with intellectual impairments they support, there is a pervasive dilemma between, on one hand, encouraging participation and, on the other, getting institutional jobs done. The authors show how the practices of encouraging talk that they describe move between the two horns of that dilemma

Promoting choice and control in residential services for people with learning disabilities

This paper discusses the gap between policy goals and practice in residential services for people with learning disabilities. Drawing on a nine month ethnographic study of three residential services, it outlines a range of obstacles to the promotion of choice and control that were routinely observed in the culture and working practices of the services. Issues discussed include conflicting service values and agendas, inspection regimes, an attention to the bigger decisions in a person's life when empowerment could more quickly and effectively be promoted at the level of everyday practice, problems of communication and interpretation and the pervasiveness of teaching. We offer a range of suggestions as to how these obstacles might be tackle

How adults with a profound intellectual disability engage others in interaction

Using video records of everyday life in a residential home, we report on what interactional practices are used by people with severe and profound intellectual disabilities to initiate encounters. There were very few initiations, and all presented difficulties to the interlocutor; one (which we call "blank recipiency") gave the interlocutor virtually no information at all on which to base a response. Only when the initiation was of a new phase in an interaction already under way (for example, the initiation of an alternative trajectory of a proposed physical move) was it likely to be successfully sustained. We show how interlocutors (support staff; the recording researcher) responded to initiations verbally, as if to neurotypical speakers - but inappropriately for people unable to comprehend, or to produce well-fitted next turns. This misreliance on ordinary speakers' conversational practices was one factor that contributed to residents abandoning the interaction in almost all cases. We discuss the dilemma confronting care workers

How adults with a profound intellectual disability engage others in interaction

Using video records of everyday life in a residential home, we report on what interactional practices are used by people with severe and profound intellectual disabilities to initiate encounters. There were very few initiations, and all presented difficulties to the interlocutor; one (which we call "blank recipiency") gave the interlocutor virtually no information at all on which to base a response. Only when the initiation was of a new phase in an interaction already under way (for example, the initiation of an alternative trajectory of a proposed physical move) was it likely to be successfully sustained. We show how interlocutors (support staff; the recording researcher) responded to initiations verbally, as if to neurotypical speakers - but inappropriately for people unable to comprehend, or to produce well-fitted next turns. This misreliance on ordinary speakers' conversational practices was one factor that contributed to residents abandoning the interaction in almost all cases. We discuss the dilemma confronting care workers

Cleanroom strategies for micro- and nano-fabricating flexible implantable neural electronics

Implantable electronic neural interfaces typically take the form of probes and are made with rigid materials such as silicon and metals. These have advantages such as compatibility with integrated microchips, simple implantation and high-density nanofabrication but tend to be lacking in terms of biointegration, biocompatibility and durability due to their mechanical rigidity. This leads to damage to the device or, more importantly, the brain tissue surrounding the implant. Flexible polymer-based probes offer superior biocompatibility in terms of surface biochemistry and better matched mechanical properties. Research which aims to bring the fabrication of electronics on flexible polymer substrates to the nano-regime is remarkably sparse, despite the push for flexible consumer electronics in the last decade or so. Cleanroom-based nanofabrication techniques such as photolithography have been used as pattern transfer methods by the semiconductor industry to produce single nanometre scale devices and are now also used for making flexible circuit boards. There is still much scope for miniaturizing flexible electronics further using photolithography, bringing the possibility of nanoscale, non-invasive, high-density flexible neural interfacing. This work explores the fabrication challenges of using photolithography and complementary techniques in a cleanroom for producing flexible electronic neural probes with nanometre-scale features

Self-powered implantable CMOS photovoltaic cell with 18.6% efficiency

Harvesters for implantable medical applications need to generate enough energy to power their loads, but their efficiency is reduced when implanted under the tissue. Conventional photovoltaic (PV) cell harvesters made with CMOS technology stack cells in series, which raises output voltage but lowers power conversion efficiency. In addition, it is difficult to assess harvester performance prior to fabrication. To address these challenges, we developed a novel parallel PV cell configuration that fully utilizes all triple-well diodes and responds efficiently to near-infrared light. Using an optimized structure, the PV cells were fabricated through standard TSMC 65-nm CMOS technology, achieving an efficiency of 18.6%, open circuit voltage of 0.45 V, and short circuit current of 1.9 mA cm −2 . These results confirm the ability of the device to generate sufficient energy even when implanted beneath the tissue. Multiphysics finite element modeling (FEM) was used to optimize the stacking structure of the CMOS PV cell, and experimental results showed a successfully delivered power density of 1.2 mW cm −2 (single cell 1.04 mm 2 ) when placed 2 mm below porcine skin. Different array configurations of six PV cells were also experimentally studied using external wire switching, demonstrating the flexibility of the PV array in delivering different output energy for various implantable devices

Control over phase separation and nucleation using a laser-tweezing potential

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a poorly understood practice. Although there are now numerous examples of control using laser-induced nucleation, the absence of physical understanding is preventing progress. Here we show that the proximity of a liquid–liquid critical point or the corresponding binodal line can be used by a laser-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in a liquid mixture using a low-power laser diode confirm the effect. Phase separation and nucleation using a laser-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter