Growth and decay of localized disturbances on a surfactant-coated spreading film

If the surface of a quiescent thin liquid film is suddenly coated by a patch of surface active material like a surfactant monolayer, the film is set in motion and begins spreading. An insoluble surfactant will rapidly attempt to coat the entire surface of the film thereby minimizing the liquid's surface tension. The shear stress that develops during the spreading process produces a maximum in surface velocity in the region where the moving film meets the quiescent layer. This region is characterized by a shock front with large interfacial curvature and a corresponding local buildup of surfactant which creates a spike in the concentration gradient. In this paper, we investigate the sensitivity of this region to infinitesimal disturbances. Accordingly, we introduce a measure of disturbance amplification and transient growth analogous to a kinetic energy that couples variations in film thickness to the surfactant concentration. These variables undergo significant amplification during the brief period in which they are convected past the downstream tip of the monolayer, where the variation in concentration gradient and surface curvature are largest. Once they migrate past this sensitive area, the perturbations weaken considerably and the system approaches a stable configuration. It appears that the localized disturbances of the type we consider here, cannot sustain asymptotic instability. Nonetheless, our study of the dynamics leading to the large transient growth clearly illustrates how the coupling of Marangoni and capillary forces work in unison to stabilize the spreading process against localized perturbations

Thinning and disturbance growth in liquid films mobilized by continuous surfactant delivery

A generalized linear stability analysis is applied to the case of a thin liquid film propelled to spread by a continuous supply of surfactant. The time-dependent base states for the film thickness and surfactant concentration give rise to a nonautonomous system describing disturbance propagation. As a first approximation, the nonautonomous operator is treated as time independent, thereby reducing the system of equations to a standard eigenvalue problem. For the range of parameters investigated, this modal approximation reveals a band of unstable modes corresponding to the growth of transverse, sinusoidal corrugations. A transient growth analysis of the fully time-dependent system, which requires the solution of an initial value problem, also signals the possibility of large disturbance growth. In both cases, significant amplification of infinitesimal disturbances can be traced to the region of the film most rapidly thinned by Marangoni stresses, which is characterized by large interfacial curvature and a sharp variation in shear stress. In contrast to previous models implementing a finite surfactant source that predict asymptotic stability, large transient growth and asymptotic instability are possible for the case of sustained surfactant release

Microfluidic detection and analysis by integration of evanescent wave sensing with thermocapillary actuation

An integrated system capable of microfluidic actuation, detection and sensing is described which combines evanescent wave sensing with thermocapillary manipulation. Liquid droplets or streams transported across the beam path of a planar thin film waveguide, which encapsulates the microheater array, induce attenuation of the propagating waveguide modes. The attenuated signal is used to monitor droplet location, dye concentration in aqueous solutions and reaction kinetics for enzymatic hydrolysis of the sugar X-galactose by beta-galactosidase

Mechanism for Spontaneous Growth of Nanopillar Arrays in Ultrathin Films Subject to a Thermal Gradient

Several groups have reported spontaneous formation of periodic pillar-like arrays in molten polymer nanofilms confined within closely spaced substrates maintained at different temperatures. These formations have been attributed to a radiation pressure instability caused by acoustic phonons. In this work, we demonstrate how variations in the thermocapillary stress along the nanofilm interface can produce significant periodic protrusions in any viscous film no matter how small the initial transverse thermal gradient. The linear stability analysis of the interface evolution equation explores an extreme limit of B\'{e}nard-Marangoni flow peculiar to films of nanoscale dimensions in which hydrostatic forces are altogether absent and deformation amplitudes are small in comparison to the pillar spacing. Finite element simulations of the full nonlinear equation are also used to examine the array pitch and growth rates beyond the linear regime. Inspection of the Lyapunov free energy as a function of time confirms that in contrast to typical cellular instabilities in macroscopically thick films, pillar-like elongations are energetically preferred in nanofilms. Provided there occurs no dewetting during film deformation, it is shown that fluid elongations continue to grow until contact with the cooler substrate is achieved. Identification of the mechanism responsible for this phenomenon may facilitate fabrication of extended arrays for nanoscale optical, photonic and biological applications.Comment: 20 pages, 9 figure

Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses

Experiments by several groups during the past decade have shown that a molten polymer nanofilm subject to a large transverse thermal gradient undergoes spontaneous formation of periodic nanopillar arrays. The prevailing explanation is that coherent reflections of acoustic phonons within the film cause a periodic modulation of the radiation pressure which enhances pillar growth. By exploring a deformational instability of particular relevance to nanofilms, we demonstrate that thermocapillary forces play a crucial role in the formation process. Analytic and numerical predictions show good agreement with the pillar spacings obtained in experiment. Simulations of the interface equation further determine the rate of pillar growth of importance to technological applications.Comment: 5 pages, 4 figure

Absolutist Words From Search Volume Data Predict State-Level Suicide Rates in the United States

Suicide continues to be a major public health issue, especially in the United States. It is a well-established fact that depression and suicidal ideation are risk factors for suicide. Drawing on recent research that shows that absolutist words (e.g., “completely,” “totally”) constitute linguistic markers of suicidal ideation, we created an online index of absolutist thinking (ATI) using search query data (i.e., Google Trends time series). Mixed-model analyses of age-adjusted suicide rates in the United States from 2004 to 2017 revealed that ATI is linked with suicides, β = 0.22, 95% CI = [0.12, 0.31], p &lt; .001, and predicts suicides within 1 year, β = 0.16, 95% CI = [0.05, 0.28], p = .006, independently of state characteristics and historical trends. It is the first time that a collective measure of absolutist thinking is used to predict real-world suicide outcomes. Therefore, the present study paves the way for novel research avenues in clinical psychological research. </jats:p

Co-radicalisation of Islamist and Nativist Extremists in Europe

Abstract A vast amount of social science research has been dedicated to the study of Islamist extremism – in particular, to uncover its psychological and structural drivers. However, the recent revival of extreme-right extremism points to the need to investigate this re-emerging phenomenon. This article highlights some of the characteristics of the extremisation of Islamism in Europe in parallel with the rise of the extremisation of right-wing extremist groups. In doing so, we explore similarities between Islamist and right-wing extremist individuals and groups. The main premise of the article is that a threat-regulation approach fails to understand the role of contextual and structural factors in the political and religious extremisation of individuals. Instead, the article claims that a reciprocal-threat model can better explain extremist violence since it is based on the idea that nativist and Islamist extremist individuals/groups are mutually threatening each other.</jats:p

The pathogen paradox: Evidence that perceived COVID-19 threat is associated with both pro- and anti-immigrant attitudes

COVID-19 pandemic, as a global threat to humanity, is likely to instigate a variety of collective responses in the society. We examined, for the first time, whether COVID-19 threat perception is related to attitudes towards Syrian immigrants in Turkey, theorizing a dual pathway whereby threat caused by the COVID-19 pandemic would relate to both pro- and anti-immigrant feelings. While drawing upon behavioral immune system theory, we expected that pathogen threat would lead to more exclusionary attitudes; relying on the common ingroup identity model, we predicted that pathogen threat would promote inclusionary attitudes through creating a common ingroup in the face of a global threat. Results from two studies using online search volume data at the province-level (N = 81) and self-report measures at the individual level (N = 294) demonstrated that perceived COVID-19 threat was directly associated with more positive attitudes towards immigrants (Study 1 and 2). Study 2 further revealed indirect positive (through a sense of common identity) and negative (through perceptions of immigrant threat) links between COVID-19 threat perception and attitudes towards immigrants. These results highlight the importance of integrating evolutionary and social identity perspectives when assessing pathogen-related threats. We draw attention to managing the public perceptions of COVID-19 threat which may mitigate the social aftermath of the pandemic.</p