arXiv subjects

Halim Kusumaatmaja

Publications and source records attributed to Halim Kusumaatmaja.

61 records · Page 4Linked to original sources

arXiv2014

Design principles for Bernal spirals and helices with tunable pitch

Using the framework of potential energy landscape theory, we describe two in silico designs for self-assembling helical colloidal superstructures based upon dipolar dumbbells and Janus-type building blocks, respectively. Helical superstructures with controllable pitch length are obtained using external magnetic field driven assembly of asymmetric dumbbells involving screened electrostatic as well as magnetic dipolar interactions. The pitch of the helix is tuned by modulating the Debye screening length over an experimentally accessible range. The second design is based on building blocks composed of rigidly linked spheres with short-range anisotropic interactions, which are predicted to self-assemble into Bernal spirals. These spirals are quite flexible, and longer helices undergo rearrangements via cooperative, hinge-like moves, in agreement with experiment.

cond-mat.soft↗

arXiv2013

Defect Motifs for Constant Mean Curvature Surfaces

The energy landscapes of electrostatically charged particles embedded on constant mean curvature surfaces are analysed for a wide range of system size, curvature, and interaction potentials. The surfaces are taken to be rigid, and the basin-hopping method is used to locate the putative global minimum structures. The defect motifs favoured by potential energy agree with experimental observations for colloidal systems: extended defects (scars and pleats) for weakly positive and negative Gaussian curvatures, and isolated defects for strongly negative Gaussian curvatures. Near the phase boundary between these regimes the two motifs are in strong competition, as evidenced from the appearance of distinct funnels in the potential energy landscape. We also report a novel defect motif consisting of pentagon pairs.

cond-mat.soft↗

arXiv2012

Anisotropic wetting and de-wetting of drops on substrates patterned with polygonal posts

We present results showing how water drops, produced by ink-jet printing, spread on surfaces patterned with lattices of diamond or triangular posts. Considering post widths typically ~7 m and lattice spacings between 15-40 m, we observe drop shapes with 3,4 and 6-fold symmetry, depending on both the symmetry of the lattice and the shape of the posts. This is a result of the different mechanisms of interface pinning and depinning which depend on the direction of the contact line motion with respect to the post shape. Lattice Boltzmann simulations are used to describe these mechanisms in detail for triangular posts. We also follow the motion of the contact line as the drops evaporate showing that they tend to return to their original shape. To explain this we show that the easy direction for movement is the same for spreading and drying drops. We compare the behaviour of small drops with that of larger drops created by jetting several drops at the same position. We find that the contact line motion is unexpectedly insensitive to drop volume, even when a spherical cap of fluid forms above the posts. The findings are relevant to microfluidic applications and to the control of drop shapes in ink-jet printing.

cond-mat.soft↗

arXiv2012

Non-isomorphic nucleation pathways arising from morphological transitions of liquid channels

Motivated by unexpected morphologies of the emerging liquid phase (channels, bulges, droplets) at the edge of thin, melting alkane terraces, we propose a new heterogeneous nucleation pathway. The competition between bulk and interfacial energies and the boundary conditions determine the growth and shape of the liquid phase at the edge of the solid alkane terraces. Calculations and experiments reveal a "pre-critical" shape transition (channel-to-bulges) of the liquid before reaching its critical volume along a putative shape-conserving path. Bulk liquid emerges from the new shape, and depending on the degree of supersaturation, the new pathway may have two, one, or zero energy barriers. The findings are broadly relevant for many heterogeneous nucleation processes because the novel pathway is induced by common, widespread surface topologies (scratches, steps, etc.).

physics.chem-ph↗

arXiv2012

Wetting-induced budding of vesicles in contact with several aqueous phases

Osmotic deflation of vesicles enclosing two liquid phases can lead to bulging of one of the phases from the vesicle body. This budding process is preceded by a complete to partial wetting transition of one of the liquid phases on the membrane and depends on the membrane tensions and the tension of the interface between the enclosed liquid phases. These tensions dominate in different morphology regimes, the crossover of which initiates the budding process. In addition, the degree of budding can be controlled by aspiration via micropipettes. We also demonstrate that the budding direction can be reversed if there are two external phases in contact with the vesicle.

cond-mat.soft↗

arXiv2011

Droplet-Induced Budding Transitions of Membranes

Motivated by recent experiments on biomimetic membranes exposed to several aqueous phases, we theoretically study the morphology of a membrane in contact with a liquid droplet formed via aqueous phase separation. We concentrate on membranes with negligible spontaneous curvature. At small droplet volume, bending energy dominates and the droplet is only partially wrapped by the membrane. At large volume, this configuration can become unstable and undergo a discontinuous transition to a state, in which the droplet is (almost) completely wrapped by the membrane. A morphology diagram, showing the parameter region where such budding transition occurs, is constructed as a function of the membrane tension and the intrinsic contact angle of the liquid with the membrane. The effects of spontaneous curvature are discussed qualitatively.

cond-mat.soft↗

arXiv2010

Equilibrium morphologies and effective spring constants of capillary bridges

We theoretically study the behaviour of a liquid bridge formed between a pair of rigid and parallel plates. The plates are smooth, they may either be homogeneous or decorated by circular patches of more hydrophilic domains, and they are generally not identical. We calculate the mechanical equilibrium distance of the liquid bridge as a function of liquid volume, contact angle and radius of the chemical domain. We show that a liquid bridge can be an equilibrium configuration as long as the sum of the contact angles at the two walls is larger than $180^\circ$. When comparisons are possible, our results agree well with recent analytical and molecular dynamics simulation results. We also derive the effective spring constant of the liquid bridge as it is perturbed from its equilibrium distance. The spring constant diverges when the sum of the contact angles is $180^\circ$ and is finite otherwise. The value of the spring constant decreases with increasing contact angle and volume, and the rate, at which it decreases, depends strongly on the properties of the two plates.

cond-mat.soft↗