Self-assembly of repulsive interfacial particles via collective sinking

Lee, D. G.,Cicuta, P.,Vella, D. Royal Society of Chemistry 2017 SOFT MATTER Vol.13 No.1

<P>Charged colloidal particles trapped at an air-water interface are well known to form an ordered crystal, stabilized by a long ranged repulsion; the details of this repulsion remain something of a mystery, but all experiments performed to date have confirmed a dipolar-repulsion, at least at dilute concentrations. More complex arrangements are often observed, especially at higher concentration, and these seem to be incompatible with a purely repulsive potential. In addition to electrostatic repulsion, interfacial particles may also interact via deformation of the surface: so-called capillary effects. Pair-wise capillary interactions are well understood, and are known to be too small (for these colloidal particles) to overcome thermal effects. Here we show that collective effects may significantly modify the simple pair-wise interactions and become important at higher density, though we remain well below close packing throughout. In particular, we show that the interaction of many interfacial particles can cause much larger interfacial deformations than do isolated particles, and show that the energy of interaction per particle due to this 'collective sinking'' grows as the number of interacting particles grows. Though some of the parameters in our simple model are unknown, the scaling behaviour is entirely consistent with experimental data, strongly indicating that estimating interaction energy based solely on pair-wise potentials may be too simplistic for surface particle layers.</P>

Optical trapping of colloidal particles and cells by focused evanescent fields using conical lenses

Yoon, Young Zoon,Cicuta, Pietro The Optical Society 2010 Optics express Vol.18 No.7

<P>We demonstrate advantages in terms of trapping force distribution and laser efficiency that come from using a telescopic pair of conical lenses ('axicon') to generate a ring-like beam, that in conjunction with a high NA objective is used for direct optical trapping with a focused evanescent field near a surface. Various field geometries are considered and compared. First, a Gaussian beam and a laser beam focused on the back focal plane of the objective are compared with each other, and they are scanned across the inlet aperture of the objective. This allows to detect the point of total internal refraction, and to study the trapping power near the surface. We confirm that the hollow beam generated by the conical lenses can generate an evanescent field after a high NA objective lens, and that micron-sized particles can be trapped stably. Finally, we apply the focused evanescent field to erythrocytes under flow, showing that cells are trapped against the flow and are held horizontally against the surface. This is a different equilibrium condition compared to conventional single beam traps, and it is particularly favorable for monitoring the cell membrane. We foresee the integration of this type of trapping with the imaging techniques based on total internal refraction fluorescence (TIRF).</P>

The nonlinear mechanical response of the red blood cell

Yoon, Young-Zoon,Kotar, Jurij,Yoon, Gilwon,Cicuta, Pietro Institute of Physics 2008 Physical Biology Vol.5 No.3

<P>We measure the dynamical mechanical properties of human red blood cells. A single cell response is measured with optical tweezers. We investigate both the stress relaxation following a fast deformation and the effect of varying the strain rate. We find a power-law decay of the stress as a function of time, down to a plateau stress, and a power-law increase of the cell's elasticity as a function of the strain rate. Interestingly, the exponents of these quantities violate the linear superposition principle, indicating a nonlinear response. We propose that this is due to the breaking of a fraction of the crosslinks during the deformation process. The soft glassy rheology model accounts for the relation between the exponents we observe experimentally. This picture is consistent with recent models of bond remodeling in the red blood cell's molecular structure. Our results imply that the blood cell's mechanical behavior depends critically on the deformation process.</P>

Mechanical properties of ternary lipid membranes near a liquid–liquid phase separation boundary

Yoon, Young Zoon,Hale, John P,Petrov, Peter G,Cicuta, Pietro IOP Pub 2010 Journal of Physics, Condensed Matter Vol.22 No.6

<P>We study the mechanical properties of ternary lipid bilayers assembled in giant vesicles, formed from a saturated and an unsaturated phosphocholine (in equal proportions) and cholesterol. As a function of temperature, these systems can undergo in-plane phase separation. Using image analysis we identify the vesicle contour, and quantify the vesicle shape and the amplitude of membrane thermal fluctuations. The two lipid compositions chosen show different thermotropic behaviours. At 60 mol% cholesterol the membrane is in a uniform liquid state over the entire temperature range investigated (10–50 °C), but vesicles containing 30 mol% cholesterol undergo phase separation into two immiscible liquid phases at around 28 °C. Upon cooling below this transition temperature we observe a marked increase in the measured bending elastic modulus. Phase separation proceeds over a long time (tens of minutes), and we measure the properties of vesicles both during the domain coarsening phase and in the fully phase separated condition. Fluorescence microscopy allows us to identify the coexisting phases. We can therefore measure directly the bending moduli of each of the phases as a function of temperature, showing a strong variation which is attributed to the changing phospholipid and cholesterol composition. </P>

Flickering Analysis of Erythrocyte Mechanical Properties: Dependence on Oxygenation Level, Cell Shape, and Hydration Level

Yoon, Y.Z.,Hong, H.,Brown, A.,Kim, D.C.,Kang, D.J.,Lew, V.L.,Cicuta, P. Biophysical Society ; Published for the Biophysica 2009 Biophysical journal Vol.97 No.6

Erythrocytes (red blood cells) play an essential role in the respiratory functions of vertebrates, carrying oxygen from lungs to tissues and CO<SUB>2</SUB> from tissues to lungs. They are mechanically very soft, enabling circulation through small capillaries. The small thermally induced displacements of the membrane provide an important tool in the investigation of the mechanics of the cell membrane. However, despite numerous studies, uncertainties in the interpretation of the data, and in the values derived for the main parameters of cell mechanics, have rendered past conclusions from the fluctuation approach somewhat controversial. Here we revisit the experimental method and theoretical analysis of fluctuations, to adapt them to the case of cell contour fluctuations, which are readily observable experimentally. This enables direct measurements of membrane tension, of bending modulus, and of the viscosity of the cell cytoplasm. Of the various factors that influence the mechanical properties of the cell, we focus here on: 1), the level of oxygenation, as monitored by Raman spectrometry; 2), cell shape; and 3), the concentration of hemoglobin. The results show that, contrary to previous reports, there is no significant difference in cell tension and bending modulus between oxygenated and deoxygenated states, in line with the softness requirement for optimal circulatory flow in both states. On the other hand, tension and bending moduli of discocyte- and spherocyte-shaped cells differ markedly, in both the oxygenated and deoxygenated states. The tension in spherocytes is much higher, consistent with recent theoretical models that describe the transitions between red blood cell shapes as a function of membrane tension. Cell cytoplasmic viscosity is strongly influenced by the hydration state. The implications of these results to circulatory flow dynamics in physiological and pathological conditions are discussed.

Biophotonic techniques for the study of malaria-infected red blood cells

Mauritz, Jakob M. A.,Esposito, Alessandro,Tiffert, Teresa,Skepper, Jeremy N.,Warley, Alice,Yoon, Young-Zoon,Cicuta, Pietro,Lew, Virgilio L.,Guck, Jochen R.,Kaminski, Clemens F. Springer-Verlag 2010 Medical & biological engineering & computing Vol.48 No.10