A double rigidity transition rules the fate of drying colloidal drops

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities. While the second instability is due to the permanent aggregation of nanoparticles, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops

Structure and rheological properties of model microemulsion networks filled with nanoparticles

Model microemulsion networks of oil droplets stabilized by non ionic surfactant and telechelic polymer C18-PEO(10k)-C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering. A Reverse Monte Carlo approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R=10nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%

Structural interventions and social suffering: Responding to amphetamine-type stimulant use among female entertainment and sex workers in Cambodia

Background: In Cambodia, HIV infection remains high among female entertainment and sex workers (FESW) and the use of amphetamine-type stimulants (ATS) is an independent risk factor for unprotected sex and sexually transmitted infections among this group. For decades public health approaches to HIV prevention in low and middle income countries (LMIC) have attempted to target the macro-power relations that shape risk behaviour with structural interventions. Recent research has highlighted that interventions that combine ATS risk reduction, in the form of financial incentives for abstinence, with existing HIV prevention programmes, may also play an important role. However, whether this approach goes far enough as a response to structural drivers of risk requires further examination. Methods: Semi-structured in-depth interviews were conducted with 30 FESW (mean age 25 years) from five provinces in Cambodia, as part of formative research for the implementation of the Cambodia Integrated HIV and Drug Prevention (CIPI) trial. The aim was to explore the contexts and drivers of ATS use. Data were analysed using grounded theory. Results: In addition to increasing occupational functionality, ATS were used to control pervasive feelings of ‘sadness’ in relation to the lived experience of poverty, family and relationship problems. Feeling sad could be viewed as an expression of social suffering, in response to competing priorities and seemingly inescapable constraints imposed by a lack of options for income generation, gender inequalities and stigma. Participants expressed interest in microenterprise (ME) opportunities, particularly vocational training, that could create new work opportunities beyond sex work and ATS use. Conclusion: In addition to reducing ATS use, HIV prevention interventions need to target sources of sadness and social suffering as drivers of risk among FESW in this context. The inclusion of ME opportunities in HIV prevention, to alleviate social suffering, warrants further investigation through qualitative and ethnographic research